feat: add CalDAV bidirectional sync service (Phase 3O)

Implements CalDAV sync using github.com/emersion/go-webdav:

- CalDAVService with background polling (configurable per-tenant interval)
- Push: deadlines -> VTODO, appointments -> VEVENT on create/update/delete
- Pull: periodic fetch from CalDAV, reconcile with local DB
- Conflict resolution: KanzlAI wins dates/status, CalDAV wins notes/description
- Conflicts logged as case_events with caldav_conflict type
- UID pattern: kanzlai-{deadline|appointment}-{uuid}@kanzlai.msbls.de
- CalDAV config per tenant in tenants.settings JSONB

Endpoints:
- POST /api/caldav/sync — trigger full sync for current tenant
- GET /api/caldav/status — last sync time, item counts, errors

8 unit tests for UID generation, parsing, path construction, config parsing.

backend/go.mod

@@ -3,11 +3,17 @@ module mgit.msbls.de/m/KanzlAI-mGMT
 go 1.25.5
 
 require (
-	github.com/anthropics/anthropic-sdk-go v1.27.1 // indirect
-	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
-	github.com/google/uuid v1.6.0 // indirect
-	github.com/jmoiron/sqlx v1.4.0 // indirect
-	github.com/lib/pq v1.12.0 // indirect
+	github.com/anthropics/anthropic-sdk-go v1.27.1
+	github.com/emersion/go-ical v0.0.0-20250609112844-439c63cef608
+	github.com/emersion/go-webdav v0.7.0
+	github.com/golang-jwt/jwt/v5 v5.3.1
+	github.com/google/uuid v1.6.0
+	github.com/jmoiron/sqlx v1.4.0
+	github.com/lib/pq v1.12.0
+)
+
+require (
+	github.com/teambition/rrule-go v1.8.2 // indirect
 	github.com/tidwall/gjson v1.18.0 // indirect
 	github.com/tidwall/match v1.1.1 // indirect
 	github.com/tidwall/pretty v1.2.1 // indirect