fix(materi): align tags column type to jsonb (schema ↔ migration mismatch)

The Drizzle schema used pgText('tags').array() which emits a Postgres
text[] column, but the migration defines tags as jsonb. The mismatch caused
INSERT/LIST on materi_documents to throw INTERNAL_SERVER_ERROR (500) because
Drizzle sent a text[] where the column expected jsonb.

- schema.ts: tags → pgJsonb('tags').notNull().default('[]')
- migration: dropped+recreated to match schema (jsonb, ms epoch defaults,
  owner_user_id default 'anonymous')

services/backend/src/shared/database/schema.ts

@@ -616,7 +616,7 @@ export const pgMateriDocumentsTable = pgTable(
     description: pgText("description"),
     content: pgText("content").notNull(),
     category: pgText("category").notNull().default("general"),
-    tags: pgText("tags").array(),
+    tags: pgJsonb("tags").notNull().default("[]"),
     owner_user_id: pgText("owner_user_id").notNull(),
     guild_id: pgText("guild_id"),
     channel_id: pgText("channel_id"),