feat: move configuration from .env to DB with Admin UI management

Replace hardcoded .env configuration with database-backed settings
manageable through the Admin web interface. This reduces .env to
bootstrap-only variables (DB, Keycloak, encryption keys).

Backend:
- Add SystemSetting Prisma model with category, valueType, isSecret
- Add system-settings NestJS module (CRUD, 60s cache, encryption)
- Refactor all 7 connectors to lazy-load credentials from DB via
  CredentialsService.findActiveByType() instead of ConfigService
- Add event-driven credential reload (@nestjs/event-emitter)
- Dynamic CORS origins and conditional Swagger from DB settings
- Fix JWT strategy: use Keycloak JWKS (RS256) instead of symmetric secret
- Add SYSTEM_SETTINGS_VIEW/MANAGE permissions
- Seed 13 default settings (sync intervals, features, branding, CORS)
- Add env-to-db migration script (prisma/migrate-env-to-db.ts)

Frontend:
- Add use-credentials hook (full CRUD for integration credentials)
- Add use-system-settings hook (read/update system settings)
- Wire admin-integrations page to real API (create/update/test/toggle)
- Add admin system-settings page with 4 tabs (Branding, CORS, Sync, Features)
- Fix sidebar double-highlighting with exactMatch flag
- Fix integration detail fallback when API unavailable
- Fix API client to unwrap backend's {success, data} envelope
- Update NEXT_PUBLIC_API_URL to include /v1 version prefix
- Fix activity-widget hydration error
- Add i18n keys for systemSettings (de + en)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/prisma/schema.prisma

@@ -654,3 +654,21 @@ model AuditLog {
   @@index([action])
   @@index([createdAt])
 }
+
+// =============================================================================
+// SYSTEM SETTINGS
+// =============================================================================
+
+model SystemSetting {
+  id          String   @id @default(cuid())
+  key         String   @unique
+  value       String
+  category    String
+  description String?
+  valueType   String   @default("string")
+  isSecret    Boolean  @default(false)
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+
+  @@index([category])
+}