feat: add batch_id/target_label to sessions and batch launch endpoint

- Add batch_id (UUID, nullable, indexed) and target_label (String 255,
  nullable) columns to the Session model
- Manual Alembic migration 6e8128ef2aa8 applies both columns
- POST /sessions/batch creates one session per target for maintenance
  flows; rejects empty targets (422) and non-maintenance trees (400)
- SessionResponse schema exposes batch_id and target_label
- 3 new integration tests, all 540 tests pass

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/schemas/session.py

@@ -78,6 +78,9 @@ class SessionResponse(BaseModel):
     def normalize_text_fields(cls, v):
         return v or ""
 
+    batch_id: Optional[UUID] = None
+    target_label: Optional[str] = None
+
     class Config:
         from_attributes = True