[interface-change] Phase 4: Extract shared types + fix Docker context

Shared types extracted to shared/types/:
- companion.ts (33+ types, was 100% duplicated admin-lehrer ↔ studio-v2)
- klausur.ts (18+ types, was 95% duplicated across 4 locations)
- ocr-labeling.ts (11 types, was 100% duplicated admin-lehrer ↔ website)

Original type files replaced with re-exports for backward compat.
tsconfig.json paths updated with @shared/* alias in all 3 services.

Docker: Changed build context from ./service to . (root) so shared/
is accessible. Dockerfiles updated to COPY service/ + shared/.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -60,8 +60,8 @@ services:
   # =========================================================
   admin-lehrer:
     build:
-      context: ./admin-lehrer
-      dockerfile: Dockerfile
+      context: .
+      dockerfile: admin-lehrer/Dockerfile
       args:
         NEXT_PUBLIC_API_URL: ${NEXT_PUBLIC_API_URL:-https://macmini:8001}
         NEXT_PUBLIC_OLD_ADMIN_URL: ${NEXT_PUBLIC_OLD_ADMIN_URL:-http://macmini:3000/admin}
@@ -95,8 +95,8 @@ services:
 
   studio-v2:
     build:
-      context: ./studio-v2
-      dockerfile: Dockerfile
+      context: .
+      dockerfile: studio-v2/Dockerfile
       args:
         NEXT_PUBLIC_VOICE_SERVICE_URL: ${NEXT_PUBLIC_VOICE_SERVICE_URL:-wss://macmini:8091}
         NEXT_PUBLIC_KLAUSUR_SERVICE_URL: ${NEXT_PUBLIC_KLAUSUR_SERVICE_URL:-https://macmini:8086}
@@ -116,8 +116,8 @@ services:
 
   website:
     build:
-      context: ./website
-      dockerfile: Dockerfile
+      context: .
+      dockerfile: website/Dockerfile
       args:
         NEXT_PUBLIC_BILLING_API_URL: ${NEXT_PUBLIC_BILLING_API_URL:-https://macmini:8083}
         NEXT_PUBLIC_APP_URL: ${NEXT_PUBLIC_APP_URL:-https://macmini}