Refactor database handling to exclusively support PostgreSQL

- Removed SQLite support from the configuration and database initialization logic.
- Updated database migration scripts to focus solely on PostgreSQL migrations.
- Simplified logging messages to reflect PostgreSQL usage.
- Adjusted database schema definitions to remove SQLite-specific types and structures.
- Modified tests to ensure compatibility with PostgreSQL, including changes to table creation and data types.
- Cleaned up unused imports and code related to SQLite.

src/muxer-queue.ts

@@ -53,7 +53,7 @@ export function getDatabase() {
   return undefined as unknown as never;
 }
 
-// ── Persistent KV store (replaces SQLite uiState table) ────────────────────
+// ── Persistent KV store ────────────────────────────────────────────────────
 
 const KV_PREFIX = "kv:";