feat(backend): implement phases 0-6 (auth, kc, wahl, files, chat, sync)

Full NestJS backend for the KC-App platform:
- auth: Authentik OIDC resource-server strategy + guest invite-code JWT
  login, plus TokenVerificationService for the WS handshake path
- kc: Leitungsteam-only KC (event) creation/listing
- wahl: Wahl/Workshop admin, Force-Zuteilung overrides, ZuteilungService
  (port of the WP plugin's kc_run_zuteilung), CSV export
- files: LT-only upload with visibility tiers; list/download filtered by
  caller tier; StorageProvider abstraction (WebDAV/Nextcloud default, S3)
- chat: Gemeinde group / DM / LT-wide / broadcast channels; REST + raw ws
  gateway sharing ChatService access rules
- sync: append-only SyncLogEntry replication log + local<->cloud
  push/pull scheduler, shared-secret guarded
- common: Role enum, @Roles decorator, KC-scoped RolesGuard (LT global)
- serves client/web/ interim static web client under / (API under /api)

Typecheck, nest build and boot test pass; needs real Postgres/Authentik/
Nextcloud to run end to end.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/package.json

@@ -21,6 +21,7 @@
     "prisma:migrate": "prisma migrate dev"
   },
   "dependencies": {
+    "@aws-sdk/client-s3": "^3.679.0",
     "@nestjs/common": "^10.4.15",
     "@nestjs/config": "^3.3.0",
     "@nestjs/core": "^10.4.15",
@@ -28,15 +29,20 @@
     "@nestjs/passport": "^10.0.3",
     "@nestjs/platform-express": "^10.4.15",
     "@nestjs/platform-ws": "^10.4.15",
+    "@nestjs/schedule": "^4.1.1",
+    "@nestjs/serve-static": "^4.0.2",
     "@nestjs/websockets": "^10.4.15",
     "@prisma/client": "^5.22.0",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.14.1",
+    "jsonwebtoken": "^9.0.2",
     "jwks-rsa": "^3.1.0",
+    "multer": "^2.0.1",
     "passport": "^0.7.0",
     "passport-jwt": "^4.0.1",
     "reflect-metadata": "^0.2.2",
     "rxjs": "^7.8.1",
+    "webdav": "^5.7.1",
     "ws": "^8.18.0"
   },
   "devDependencies": {
@@ -45,6 +51,8 @@
     "@nestjs/testing": "^10.4.15",
     "@types/express": "^4.17.21",
     "@types/jest": "^29.5.14",
+    "@types/jsonwebtoken": "^9.0.7",
+    "@types/multer": "^1.4.12",
     "@types/node": "^20.17.9",
     "@types/passport": "^1.0.17",
     "@types/passport-jwt": "^4.0.1",
@@ -67,13 +75,19 @@
     "typescript": "^5.6.3"
   },
   "jest": {
-    "moduleFileExtensions": ["js", "json", "ts"],
+    "moduleFileExtensions": [
+      "js",
+      "json",
+      "ts"
+    ],
     "rootDir": "src",
     "testRegex": ".*\\.spec\\.ts$",
     "transform": {
       "^.+\\.(t|j)s$": "ts-jest"
     },
-    "collectCoverageFrom": ["**/*.(t|j)s"],
+    "collectCoverageFrom": [
+      "**/*.(t|j)s"
+    ],
     "coverageDirectory": "../coverage",
     "testEnvironment": "node"
   }