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>

apps/aelis-backend/src/sources/errors.ts

@@ -0,0 +1,32 @@
+/**
+ * Thrown by a FeedSourceProvider when the source is not enabled for a user.
+ *
+ * UserSessionManager's Promise.allSettled handles this gracefully —
+ * the source is excluded from the session without crashing.
+ */
+export class SourceDisabledError extends Error {
+	readonly sourceId: string
+	readonly userId: string
+
+	constructor(sourceId: string, userId: string) {
+		super(`Source "${sourceId}" is not enabled for user "${userId}"`)
+		this.name = "SourceDisabledError"
+		this.sourceId = sourceId
+		this.userId = userId
+	}
+}
+
+/**
+ * Thrown when an operation targets a user source that doesn't exist.
+ */
+export class SourceNotFoundError extends Error {
+	readonly sourceId: string
+	readonly userId: string
+
+	constructor(sourceId: string, userId: string) {
+		super(`Source "${sourceId}" not found for user "${userId}"`)
+		this.name = "SourceNotFoundError"
+		this.sourceId = sourceId
+		this.userId = userId
+	}
+}