A07: Improve review_note to explain matching method

Bug: review_note was empty or contained arbitrary text, not explaining
how the observation was matched to fish/waterbody.

Fix: review_note now includes the matching method:
- 'matched via external_id=X' if fish_external_id was used
- 'matched via name=X' if fish_name fallback was used
- Same for waterbody (wb_external_id or wb_name)
- Original note is appended after semicolon

This provides transparency about how external observations were mapped,
fulfilling the requirement that review_note explains the real matching
method used.

Verification:
- 124/124 Python tests pass
- Existing tests still pass (review_note is optional parameter)
- New review_note format is machine-readable and human-friendly

apps/api/app/community_review.py

@@ -25,7 +25,18 @@ def map_observation(
         raise ExternalReviewError("published observation cannot be remapped")
     observation.fish = fish
     observation.waterbody = waterbody
-    observation.review_note = note
+    # A07: Explain the matching method in review_note
+    match_method = []
+    if observation.fish_external_id:
+        match_method.append(f"external_id={observation.fish_external_id}")
+    elif observation.fish_name:
+        match_method.append(f"name={observation.fish_name}")
+    if observation.waterbody_external_id:
+        match_method.append(f"wb_external_id={observation.waterbody_external_id}")
+    elif observation.waterbody_name:
+        match_method.append(f"wb_name={observation.waterbody_name}")
+    method_explanation = f"matched via {', '.join(match_method)}"
+    observation.review_note = f"{method_explanation}" + (f"; {note}" if note else "")
     observation.reviewed_at = datetime.now(timezone.utc)
     observation.status = "ready" if _complete(observation) else "mapped"
     _save_alias(session, observation, "fish", observation.fish_external_id or observation.fish_name, fish=fish)