fix: switch articles_fts to external-content FTS5 (closes #48)

[thunpisit]

Summary

Closes #48. Migrates `articles_fts` from a contentless to an external-content FTS5 virtual table. The contentless pattern from migration 0002 required `('delete', old.rowid, …all column values…)` tuples to match exactly — any drift broke UPDATE and DELETE on `article_localizations` with `SQLITE_ERROR`.

What changed

Migration 0011 (`drizzle/0011_fts5_external_content.sql`):

1. Drop the three existing triggers + the old virtual table.
2. Recreate as external-content with `content = 'article_localizations', content_rowid = 'rowid'`.
3. `INSERT INTO articles_fts(articles_fts) VALUES('rebuild')` to populate from the linked table.
4. New triggers use the simpler delete pattern that needs only rowid:
   ```sql
   INSERT INTO articles_fts(articles_fts, rowid) VALUES('delete', old.rowid);
   ```

Why external-content is the right fix

• Drift becomes impossible. Delete only references rowid; nothing to match against.
• Recovery in one statement. `articles_fts(articles_fts) VALUES('rebuild')` rebuilds the index from the source table without a DROP/CREATE.
• Same query surface. `SELECT … FROM articles_fts WHERE articles_fts MATCH ?` keeps working — the application code doesn't change.

Live verification

Migration applied to khaopad-example-db. Reproduced the original bug pattern:

```sql
INSERT INTO article_localizations (...) VALUES (...); -- ok before fix
UPDATE article_localizations SET ... WHERE id='...'; -- threw SQLITE_ERROR before
DELETE FROM article_localizations WHERE id='...'; -- threw SQLITE_ERROR before
```

After the fix: all three succeed. `SELECT … FROM articles_fts WHERE articles_fts MATCH 'khao'` still returns ranked snippets.

Migration notes

• Migration 0011 already applied to live khaopad-example-db.
• Downstream installs need to apply 0011 via `pnpm run db:migrate:remote` (or equivalent).
• The migration is idempotent within itself (uses `DROP IF EXISTS` / `CREATE IF NOT EXISTS`) so re-running is safe.
• No data loss — the `rebuild` in step 3 reads from `article_localizations` (source of truth, untouched).

Test plan

• Migration applies cleanly on live D1
• INSERT / UPDATE / DELETE all succeed on `article_localizations` after fix
• `articles_fts MATCH` still returns ranked + snippet results
• `pnpm run build` succeeds
• After deploy: edit an existing article via the CMS → save succeeds
• After deploy: delete an article via the CMS → succeeds

🤖 Generated with Claude Code