Fix collation inconsistency on saved_filters/ticket_attachments (#41)

README Dev Note #12 mandates utf8mb4_general_ci for new tables, but
these two tables were created with utf8mb4_unicode_ci in the baseline
schema — inconsistent with every other table, and a future join or
comparison against a general_ci column would need explicit COLLATE
casts or hit "Illegal mix of collations" errors.

- Fixed 000_baseline.sql so a fresh install matches the convention
  directly.
- Added 002_fix_collation_consistency.sql for existing deployments.
  MariaDB silently drops the inline CHECK (json_valid(...)) constraint
  on saved_filters.filter_criteria when that column is MODIFYed (found
  by actually running this against a local MariaDB instance), so the
  migration explicitly re-adds it after the collation conversion.

Verified against a local MariaDB 10.11: baseline applies cleanly,
migration is idempotent (safe to run twice), and the json_valid CHECK
is still enforced afterward.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X

migrations/000_baseline.sql

@@ -155,7 +155,7 @@ CREATE TABLE IF NOT EXISTS `saved_filters` (
   UNIQUE KEY `unique_user_filter_name` (`user_id`,`filter_name`),
   KEY `idx_user_filters` (`user_id`,`is_default`),
   CONSTRAINT `saved_filters_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`user_id`) ON DELETE CASCADE
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
 
 -- ============ status_transitions ============
 CREATE TABLE IF NOT EXISTS `status_transitions` (
@@ -185,7 +185,7 @@ CREATE TABLE IF NOT EXISTS `ticket_attachments` (
   KEY `idx_attachments_ticket` (`ticket_id`),
   KEY `idx_attachments_uploaded_by` (`uploaded_by`),
   CONSTRAINT `ticket_attachments_ibfk_1` FOREIGN KEY (`uploaded_by`) REFERENCES `users` (`user_id`) ON DELETE SET NULL
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
 
 -- ============ ticket_comments ============
 CREATE TABLE IF NOT EXISTS `ticket_comments` (