fix: add ResolutionFlow service account to own default tree steps in library

Default/system trees had no author_id (NULL), causing a NOT NULL violation
when syncing steps to step_library.created_by on publish.

- Add is_service_account flag to users table (migration 4f4137ce)
- Add service_account.py: idempotent ensure_service_account() creates
  noreply@resolutionflow.com with unusable password on startup
- Cache service account ID on app.state at lifespan startup
- Add get_service_account_id() FastAPI dep (returns None in tests)
- sync_steps_from_tree: resolve author_id or service_account_id as created_by
- create_tree: set author_id=service_account_id for is_default trees
- Migration 1490781700bc: backfill author_id on 31 existing default trees

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

backend/app/main.py

@@ -14,6 +14,7 @@ from app.core.middleware import RequestLoggingMiddleware, ErrorLoggingMiddleware
 from app.core.rate_limit import limiter
 from app.api.router import api_router
 from app.core.scheduler import scheduler, load_all_schedules, _cleanup_expired_ai_conversations
+from app.core.service_account import ensure_service_account
 
 # Initialize logging configuration
 setup_logging()
@@ -103,6 +104,12 @@ async def lifespan(app: FastAPI):
     # Note: In production, use Alembic migrations instead of init_db
     # await init_db()
 
+    # Ensure service account exists and cache its ID for sync operations
+    async with async_session_maker() as db:
+        service_account_id = await ensure_service_account(db)
+    app.state.service_account_id = service_account_id
+    logger.info(f"[service_account] Service account ready (id={service_account_id})")
+
     # Start maintenance schedule runner + AI conversation cleanup
     scheduler.start()
     async with async_session_maker() as db: