feat(klausur): Handschrift entfernen + Klausur-HTR implementiert

Feature 1: Handschrift entfernen via OCR-Pipeline Session
- services/handwriting_detection.py: _detect_pencil() + target_ink Parameter
  ("all" | "colored" | "pencil") für gezielte Tinten-Erkennung
- ocr_pipeline_session_store.py: clean_png + handwriting_removal_meta Spalten
  (idempotentes ALTER TABLE in init_ocr_pipeline_tables)
- ocr_pipeline_api.py: POST /sessions/{id}/remove-handwriting Endpoint
  + "clean" zu valid_types für Image-Serving hinzugefügt

Feature 2: Klausur-HTR (Hochwertige Handschriftenerkennung)
- handwriting_htr_api.py: Neuer Router /api/v1/htr/recognize + /recognize-session
  Primary: qwen2.5vl:32b via Ollama, Fallback: trocr-large-handwritten
- services/trocr_service.py: size Parameter (base | large) für get_trocr_model()
  + run_trocr_ocr() - unterstützt jetzt trocr-large-handwritten
- main.py: HTR Router registriert

Config:
- docker-compose.yml: OLLAMA_HTR_MODEL, HTR_FALLBACK_MODEL
- .env.example: HTR Env-Vars dokumentiert

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

klausur-service/backend/ocr_pipeline_session_store.py

@@ -60,6 +60,13 @@ async def init_ocr_pipeline_tables():
         else:
             logger.debug("OCR pipeline tables already exist")
 
+        # Ensure new columns exist (idempotent ALTER TABLE)
+        await conn.execute("""
+            ALTER TABLE ocr_pipeline_sessions
+            ADD COLUMN IF NOT EXISTS clean_png BYTEA,
+            ADD COLUMN IF NOT EXISTS handwriting_removal_meta JSONB
+        """)
+
 
 # =============================================================================
 # SESSION CRUD
@@ -111,6 +118,7 @@ async def get_session_image(session_id: str, image_type: str) -> Optional[bytes]
         "deskewed": "deskewed_png",
         "binarized": "binarized_png",
         "dewarped": "dewarped_png",
+        "clean": "clean_png",
     }
     column = column_map.get(image_type)
     if not column:
@@ -135,11 +143,12 @@ async def update_session_db(session_id: str, **kwargs) -> Optional[Dict[str, Any
     allowed_fields = {
         'name', 'filename', 'status', 'current_step',
         'original_png', 'deskewed_png', 'binarized_png', 'dewarped_png',
+        'clean_png', 'handwriting_removal_meta',
         'deskew_result', 'dewarp_result', 'column_result', 'row_result',
         'word_result', 'ground_truth', 'auto_shear_degrees',
     }
 
-    jsonb_fields = {'deskew_result', 'dewarp_result', 'column_result', 'row_result', 'word_result', 'ground_truth'}
+    jsonb_fields = {'deskew_result', 'dewarp_result', 'column_result', 'row_result', 'word_result', 'ground_truth', 'handwriting_removal_meta'}
 
     for key, value in kwargs.items():
         if key in allowed_fields: