feat: duplicate link action, watcher migration, fulltext search migration

- CreateTicketView: "Link as duplicate" button on each duplicate result;
  stores chosen ticket ID in hidden field, auto-creates duplicates dependency
  after ticket is saved (TicketController)
- migrations/004: ticket_watchers table (ticket_id, user_id primary key)
- migrations/005: FULLTEXT index on tickets(title, description) for fast
  relevance search replacing LIKE scan

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

migrations/004_add_ticket_watchers.sql

@@ -0,0 +1,10 @@
+-- Migration: Add ticket watchers table
+-- Allows users to subscribe to ticket updates and receive Matrix notifications.
+
+CREATE TABLE IF NOT EXISTS ticket_watchers (
+    ticket_id  INT          NOT NULL,
+    user_id    INT          NOT NULL,
+    created_at TIMESTAMP    NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (ticket_id, user_id),
+    INDEX idx_watcher_user (user_id)
+);