fix: defensive evaluation import to prevent startup crash (#15)

Problem: Backend crashed on startup due to evaluation import failure
Solution: Wrap evaluation_helper import in try/except

Changes:
- Import evaluation_helper with error handling
- Add EVALUATION_AVAILABLE flag
- All evaluation calls now check flag before executing
- System remains functional even if evaluation system unavailable

This prevents backend crashes if:
- Migrations haven't run yet
- Dependencies are missing
- Import errors occur

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

backend/routers/activity.py

@@ -16,7 +16,15 @@ from auth import require_auth, check_feature_access, increment_feature_usage
 from models import ActivityEntry
 from routers.profiles import get_pid
 from feature_logger import log_feature_usage
-from evaluation_helper import evaluate_and_save_activity
+
+# Evaluation import with error handling (Phase 1.2)
+try:
+    from evaluation_helper import evaluate_and_save_activity
+    EVALUATION_AVAILABLE = True
+except Exception as e:
+    logger.warning(f"[AUTO-EVAL] Evaluation system not available: {e}")
+    EVALUATION_AVAILABLE = False
+    evaluate_and_save_activity = None
 
 router = APIRouter(prefix="/api/activity", tags=["activity"])
 logger = logging.getLogger(__name__)
@@ -66,6 +74,7 @@ def create_activity(e: ActivityEntry, x_profile_id: Optional[str]=Header(default
              d['rpe'],d['source'],d['notes']))
 
         # Phase 1.2: Auto-evaluation after INSERT
+        if EVALUATION_AVAILABLE:
             # Load the activity data to evaluate
             cur.execute("""
                 SELECT id, profile_id, date, training_type_id, duration_min,
@@ -102,6 +111,7 @@ def update_activity(eid: str, e: ActivityEntry, x_profile_id: Optional[str]=Head
                      list(d.values())+[eid,pid])
 
         # Phase 1.2: Auto-evaluation after UPDATE
+        if EVALUATION_AVAILABLE:
             # Load the updated activity data to evaluate
             cur.execute("""
                 SELECT id, profile_id, date, training_type_id, duration_min,
@@ -257,6 +267,7 @@ def bulk_categorize_activities(
         updated_count = cur.rowcount
 
         # Phase 1.2: Auto-evaluation after bulk categorization
+        if EVALUATION_AVAILABLE:
             # Load all activities that were just updated and evaluate them
             cur.execute("""
                 SELECT id, profile_id, date, training_type_id, duration_min,
@@ -369,7 +380,7 @@ async def import_activity_csv(file: UploadFile=File(...), x_profile_id: Optional
                     skipped += 1  # Count as skipped (not newly inserted)
 
                     # Phase 1.2: Auto-evaluation after CSV import UPDATE
-                    if training_type_id:
+                    if EVALUATION_AVAILABLE and training_type_id:
                         try:
                             # Build activity dict for evaluation
                             activity_dict = {
@@ -408,7 +419,7 @@ async def import_activity_csv(file: UploadFile=File(...), x_profile_id: Optional
                     inserted+=1
 
                     # Phase 1.2: Auto-evaluation after CSV import INSERT
-                    if training_type_id:
+                    if EVALUATION_AVAILABLE and training_type_id:
                         try:
                             # Build activity dict for evaluation
                             activity_dict = {