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/002_fix_collation_consistency.sql

@@ -0,0 +1,30 @@
+-- Fix collation inconsistency on saved_filters and ticket_attachments
+--
+-- README.md Developer Notes #12: "Database collation: Use
+-- utf8mb4_general_ci (not unicode_ci) for new tables." These two tables
+-- were created with utf8mb4_unicode_ci instead, inconsistent with every
+-- other table in the schema. Mixed collations don't break anything by
+-- themselves, but any future query joining/comparing these columns
+-- against general_ci columns needs explicit COLLATE casts or hits
+-- "Illegal mix of collations" errors.
+--
+-- Safe to re-run.
+
+ALTER TABLE `saved_filters`
+    CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;
+
+-- saved_filters.filter_criteria is pinned to utf8mb4_bin (for the
+-- json_valid() CHECK constraint) — restore that after the table-wide
+-- CONVERT TO above, which resets it to general_ci. MariaDB drops the
+-- inline CHECK when the column is MODIFYed, so re-add it explicitly.
+ALTER TABLE `saved_filters`
+    MODIFY COLUMN `filter_criteria` longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL;
+
+ALTER TABLE `saved_filters`
+    DROP CONSTRAINT IF EXISTS `saved_filters_filter_criteria_json`;
+
+ALTER TABLE `saved_filters`
+    ADD CONSTRAINT `saved_filters_filter_criteria_json` CHECK (json_valid(`filter_criteria`));
+
+ALTER TABLE `ticket_attachments`
+    CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;