feat(backend): add DB persistence layer (#79)

* feat(backend): add DB persistence layer

Replace raw pg Pool with Drizzle ORM backed by Bun.sql.
Add per-user source configuration table (user_sources).
Migrate Better Auth to drizzle-adapter.
Add AES-256-GCM credential encryption.

Co-authored-by: Ona <no-reply@ona.com>

* fix(backend): set updatedAt explicitly in all mutations

onConflictDoUpdate bypasses Drizzle's $onUpdate hook.
Set updatedAt explicitly in all mutation methods.

Co-authored-by: Ona <no-reply@ona.com>

* fix(backend): add composite index on user_sources

Add (user_id, enabled) index for the enabled() query path.

Co-authored-by: Ona <no-reply@ona.com>

---------

Co-authored-by: Ona <no-reply@ona.com>

apps/aelis-backend/src/location/provider.ts

@@ -0,0 +1,25 @@
+import { LocationSource } from "@aelis/source-location"
+
+import type { Database } from "../db/index.ts"
+import type { FeedSourceProvider } from "../session/feed-source-provider.ts"
+
+import { SourceDisabledError } from "../sources/errors.ts"
+import { sources } from "../sources/user-sources.ts"
+
+export class LocationSourceProvider implements FeedSourceProvider {
+	private readonly db: Database
+
+	constructor(db: Database) {
+		this.db = db
+	}
+
+	async feedSourceForUser(userId: string): Promise<LocationSource> {
+		const row = await sources(this.db, userId).find("aelis.location")
+
+		if (!row || !row.enabled) {
+			throw new SourceDisabledError("aelis.location", userId)
+		}
+
+		return new LocationSource()
+	}
+}