diff --git a/api/notifications.php b/api/notifications.php
index e5c0586..ff22d70 100644
--- a/api/notifications.php
+++ b/api/notifications.php
@@ -138,7 +138,7 @@ $statusSql = "SELECT DISTINCT
     COALESCE(u.display_name, u.username, 'System') AS actor_name
   FROM audit_log al
   LEFT JOIN users u ON al.user_id = u.user_id
-  INNER JOIN ticket_watchers tw ON tw.ticket_id = CAST(al.entity_id AS UNSIGNED) AND tw.user_id = ?
+  INNER JOIN ticket_watchers tw ON tw.ticket_id = al.entity_id AND tw.user_id = ?
   WHERE al.action_type = 'update'
     AND al.entity_type = 'ticket'
     AND al.user_id != ?
diff --git a/api/watch_ticket.php b/api/watch_ticket.php
index 10c7f3e..40e9867 100644
--- a/api/watch_ticket.php
+++ b/api/watch_ticket.php
@@ -12,40 +12,43 @@ require_once dirname(__DIR__) . '/models/TicketModel.php';
 
 $data = json_decode(file_get_contents('php://input'), true) ?? [];
 
-$ticketId = isset($_GET['ticket_id'])
-    ? (int)$_GET['ticket_id']
-    : (int)($data['ticket_id'] ?? 0);
+$ticketIdRaw = isset($_GET['ticket_id']) ? $_GET['ticket_id'] : ($data['ticket_id'] ?? '');
 
 if ($_SERVER['REQUEST_METHOD'] === 'POST') {
-    $ticketId = (int)($data['ticket_id'] ?? 0);
-    $action   = $data['action'] ?? '';
+    $ticketIdRaw = $data['ticket_id'] ?? '';
+    $action      = $data['action'] ?? '';
 
-    if ($ticketId <= 0 || !in_array($action, ['watch', 'unwatch'], true)) {
+    if ($ticketIdRaw === '' || !in_array($action, ['watch', 'unwatch'], true)) {
         http_response_code(400);
         echo json_encode(['success' => false, 'error' => 'Invalid parameters']);
         exit;
     }
 
     $ticketModel = new TicketModel($conn);
-    $ticket = $ticketModel->getTicketById($ticketId);
+    $ticket = $ticketModel->getTicketById((string)$ticketIdRaw);
     if (!$ticket || !$ticketModel->canUserAccessTicket($ticket, $currentUser)) {
         http_response_code(404);
         echo json_encode(['success' => false, 'error' => 'Ticket not found']);
         exit;
     }
 
+    // Use the canonical ticket_id string from the fetched ticket row, not the
+    // raw request value, so ticket_watchers always stores exactly what's in
+    // tickets.ticket_id.
+    $ticketId = $ticket['ticket_id'];
+
     if ($action === 'watch') {
         $stmt = $conn->prepare(
             "INSERT IGNORE INTO ticket_watchers (ticket_id, user_id) VALUES (?, ?)"
         );
-        $stmt->bind_param("ii", $ticketId, $userId);
+        $stmt->bind_param("si", $ticketId, $userId);
         $stmt->execute();
         $stmt->close();
     } else {
         $stmt = $conn->prepare(
             "DELETE FROM ticket_watchers WHERE ticket_id = ? AND user_id = ?"
         );
-        $stmt->bind_param("ii", $ticketId, $userId);
+        $stmt->bind_param("si", $ticketId, $userId);
         $stmt->execute();
         $stmt->close();
     }
@@ -54,7 +57,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     $countStmt = $conn->prepare(
         "SELECT COUNT(*) as cnt FROM ticket_watchers WHERE ticket_id = ?"
     );
-    $countStmt->bind_param("i", $ticketId);
+    $countStmt->bind_param("s", $ticketId);
     $countStmt->execute();
     $count = (int)$countStmt->get_result()->fetch_assoc()['cnt'];
     $countStmt->close();
@@ -73,7 +76,7 @@ if ($_SERVER['REQUEST_METHOD'] !== 'GET') {
     exit;
 }
 
-if ($ticketId <= 0) {
+if ($ticketIdRaw === '') {
     http_response_code(400);
     echo json_encode(['success' => false, 'error' => 'ticket_id required']);
     exit;
@@ -83,17 +86,19 @@ if ($ticketId <= 0) {
 // restricted ticket's watcher list and count aren't disclosed (the POST path
 // already checks this).
 $ticketModel = new TicketModel($conn);
-$ticket = $ticketModel->getTicketById($ticketId);
+$ticket = $ticketModel->getTicketById((string)$ticketIdRaw);
 if (!$ticket || !$ticketModel->canUserAccessTicket($ticket, $currentUser)) {
     http_response_code(404);
     echo json_encode(['success' => false, 'error' => 'Ticket not found']);
     exit;
 }
 
+$ticketId = $ticket['ticket_id'];
+
 $watchingStmt = $conn->prepare(
     "SELECT COUNT(*) as cnt FROM ticket_watchers WHERE ticket_id = ? AND user_id = ?"
 );
-$watchingStmt->bind_param("ii", $ticketId, $userId);
+$watchingStmt->bind_param("si", $ticketId, $userId);
 $watchingStmt->execute();
 $watching = (bool)$watchingStmt->get_result()->fetch_assoc()['cnt'];
 $watchingStmt->close();
@@ -107,7 +112,7 @@ $watchersStmt = $conn->prepare(
       ORDER BY tw.created_at ASC
       LIMIT 6"
 );
-$watchersStmt->bind_param("i", $ticketId);
+$watchersStmt->bind_param("s", $ticketId);
 $watchersStmt->execute();
 $watchersResult = $watchersStmt->get_result();
 $watchers = [];
@@ -118,7 +123,7 @@ $watchersStmt->close();
 
 // True watcher count (the list above is capped at 6 for the avatar group)
 $countStmt = $conn->prepare("SELECT COUNT(*) AS cnt FROM ticket_watchers WHERE ticket_id = ?");
-$countStmt->bind_param("i", $ticketId);
+$countStmt->bind_param("s", $ticketId);
 $countStmt->execute();
 $count = (int)$countStmt->get_result()->fetch_assoc()['cnt'];
 $countStmt->close();
diff --git a/helpers/NotificationHelper.php b/helpers/NotificationHelper.php
index 1afceac..af4cf91 100644
--- a/helpers/NotificationHelper.php
+++ b/helpers/NotificationHelper.php
@@ -204,9 +204,9 @@ class NotificationHelper
                 return;
             }
             if ($excludeUserId !== null) {
-                $stmt->bind_param("ii", $ticketId, $excludeUserId);
+                $stmt->bind_param("si", $ticketId, $excludeUserId);
             } else {
-                $stmt->bind_param("i", $ticketId);
+                $stmt->bind_param("s", $ticketId);
             }
             $stmt->execute();
             $result = $stmt->get_result();
diff --git a/migrations/000_baseline.sql b/migrations/000_baseline.sql
index f5ffec9..02781a9 100644
--- a/migrations/000_baseline.sql
+++ b/migrations/000_baseline.sql
@@ -243,11 +243,12 @@ CREATE TABLE IF NOT EXISTS `ticket_templates` (
 
 -- ============ ticket_watchers ============
 CREATE TABLE IF NOT EXISTS `ticket_watchers` (
-  `ticket_id` int(11) NOT NULL,
+  `ticket_id` varchar(9) NOT NULL,
   `user_id` int(11) NOT NULL,
   `created_at` timestamp NOT NULL DEFAULT current_timestamp(),
   PRIMARY KEY (`ticket_id`,`user_id`),
-  KEY `idx_watcher_user` (`user_id`)
+  KEY `idx_watcher_user` (`user_id`),
+  CONSTRAINT `fk_watchers_ticket_id` FOREIGN KEY (`ticket_id`) REFERENCES `tickets` (`ticket_id`) ON DELETE CASCADE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
 
 -- ============ tickets ============
diff --git a/migrations/004_fix_ticket_watchers_type.sql b/migrations/004_fix_ticket_watchers_type.sql
new file mode 100644
index 0000000..c3d4d7e
--- /dev/null
+++ b/migrations/004_fix_ticket_watchers_type.sql
@@ -0,0 +1,28 @@
+-- Fix ticket_watchers.ticket_id type mismatch and missing FK to tickets
+--
+-- ticket_watchers.ticket_id was int(11), while every other satellite table
+-- (ticket_comments, ticket_attachments, ticket_dependencies,
+-- custom_field_values) stores it as varchar(9)/varchar(10) matching
+-- tickets.ticket_id. There was also no FK constraint at all, unlike every
+-- other satellite table, so orphaned watcher rows could never be caught by
+-- referential integrity. Ticket IDs are always 9-digit numeric strings
+-- (see TicketModel::create's sprintf('%09d', ...)), so the int -> varchar(9)
+-- conversion below is lossless for real data.
+--
+-- Safe to re-run.
+
+-- Remove any watcher rows that no longer point at a real ticket (possible
+-- today precisely because there was no FK to prevent it) before adding the
+-- constraint, since orphans would make the ADD CONSTRAINT below fail.
+DELETE tw FROM `ticket_watchers` tw
+    LEFT JOIN `tickets` t ON tw.`ticket_id` = t.`ticket_id`
+    WHERE t.`ticket_id` IS NULL;
+
+ALTER TABLE `ticket_watchers`
+    MODIFY COLUMN `ticket_id` varchar(9) NOT NULL;
+
+ALTER TABLE `ticket_watchers`
+    DROP FOREIGN KEY IF EXISTS `fk_watchers_ticket_id`;
+
+ALTER TABLE `ticket_watchers`
+    ADD CONSTRAINT `fk_watchers_ticket_id` FOREIGN KEY (`ticket_id`) REFERENCES `tickets` (`ticket_id`) ON DELETE CASCADE;