feat: add CalDAV source provider (#111)

Wire CalDavSourceProvider into the backend to support CalDAV
calendar sources (e.g. iCloud) with basic auth. Config accepts
serverUrl, username, lookAheadDays, and timeZone. Credentials
(app-specific password) are stored encrypted via the existing
credential storage infrastructure.

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

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

@@ -0,0 +1,53 @@
+import { CalDavSource } from "@aelis/source-caldav"
+import { type } from "arktype"
+
+import type { FeedSourceProvider } from "../session/feed-source-provider.ts"
+
+import { InvalidSourceCredentialsError } from "../sources/errors.ts"
+
+const caldavConfig = type({
+	"+": "reject",
+	serverUrl: "string",
+	username: "string",
+	"lookAheadDays?": "number",
+	"timeZone?": "string",
+})
+
+const caldavCredentials = type({
+	"+": "reject",
+	password: "string",
+})
+
+export class CalDavSourceProvider implements FeedSourceProvider {
+	readonly sourceId = "aelis.caldav"
+	readonly configSchema = caldavConfig
+
+	async feedSourceForUser(
+		_userId: string,
+		config: unknown,
+		credentials: unknown,
+	): Promise<CalDavSource> {
+		const parsed = caldavConfig(config)
+		if (parsed instanceof type.errors) {
+			throw new Error(`Invalid CalDAV config: ${parsed.summary}`)
+		}
+
+		if (!credentials) {
+			throw new InvalidSourceCredentialsError("aelis.caldav", "No CalDAV credentials configured")
+		}
+
+		const creds = caldavCredentials(credentials)
+		if (creds instanceof type.errors) {
+			throw new InvalidSourceCredentialsError("aelis.caldav", creds.summary)
+		}
+
+		return new CalDavSource({
+			serverUrl: parsed.serverUrl,
+			authMethod: "basic",
+			username: parsed.username,
+			password: creds.password,
+			lookAheadDays: parsed.lookAheadDays,
+			timeZone: parsed.timeZone,
+		})
+	}
+}