feat: add SessionResolutionOutput model and register all 4 branching models

Introduces the session_resolution_outputs table for the three resolve
deliverables (psa_ticket_notes, knowledge_base, client_summary) with
UNIQUE(session_id, output_type) for safe upsert on regeneration.
Also registers SessionBranch, ForkPoint, SessionHandoff, and
SessionResolutionOutput in models/__init__.py so Alembic and the app
pick them up automatically.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -50,6 +50,10 @@ from .psa_activity_log import PsaActivityLog
 from .file_upload import FileUpload
 from .ai_session_embedding import AISessionEmbedding
 from .beta_feedback import BetaFeedback
+from .session_branch import SessionBranch
+from .fork_point import ForkPoint
+from .session_handoff import SessionHandoff
+from .session_resolution_output import SessionResolutionOutput
 
 __all__ = [
     "User",
@@ -114,4 +118,8 @@ __all__ = [
     "FileUpload",
     "AISessionEmbedding",
     "BetaFeedback",
+    "SessionBranch",
+    "ForkPoint",
+    "SessionHandoff",
+    "SessionResolutionOutput",
 ]