correlator: delete stale scenes on re-extraction instead of leaving them to rot (#96/#114)

Review of #132 found a blocker: PATCH /calls/{id}/transcript wipes tags/severity/location/units/embedding before re-extraction, but not the new scenes map, and doc_set(merge=True) can only add/overwrite nested map keys, never remove one. A call corrected from 3 scenes to 1 kept scenes.1/scenes.2 with pre-correction transcripts and incident_ids forever -- corrupting the exact per-scene tally #96 exists to make trustworthy, and able to re-feed stale text into #114's summarizer fix if a stale scene's incident_id still names a real incident.

Fix: fstore.doc_update(...,{"scenes": fstore.DELETE_FIELD}) -- a real delete, not a merge over an empty map. Added fstore.DELETE_FIELD (re-exports the real firebase_admin sentinel) and stubbed it in the sandboxed test conftest, which didn't have it. Also softened an overclaiming docstring: the Firestore nested-merge behavior is verified against the doc_set wrapper's pass-through, not against live Firestore.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Claude-Session: https://claude.ai/code/session_01Tbknwttzou4s46PAykmtix

drb-c2-core/tests/conftest.py

@@ -34,6 +34,10 @@ except ModuleNotFoundError:
     # into dicts that tests compare against, and a MagicMock compares unequal
     # to itself across attribute accesses.
     _fs.SERVER_TIMESTAMP = "__SERVER_TIMESTAMP__"
+    # Same reasoning as SERVER_TIMESTAMP above: a distinct sentinel, not a
+    # MagicMock, so `fstore.DELETE_FIELD is fs.DELETE_FIELD` and dict/`is`
+    # comparisons against it in tests (server-26#96/#114, PR #132) behave.
+    _fs.DELETE_FIELD = "__DELETE_FIELD__"
 
     _auth = ModuleType("firebase_admin.auth")
     _auth.verify_id_token = MagicMock()