[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>

admin-lehrer/tsconfig.json

@@ -24,6 +24,9 @@
     "paths": {
       "@/*": [
         "./*"
+      ],
+      "@shared/*": [
+        "../shared/*"
       ]
     },
     "target": "ES2017"