Implement database-backed scheduler lock for multi-worker safety

Enforce single-executor safety regardless of process launcher through a
robust database-backed lock mechanism that works reliably in container
orchestration environments.

Key changes:
1. Add scheduler_lock table to database schema (migration 4)
   - Singleton row (id=1) prevents concurrent execution
   - Stores PID, hostname, creation timestamp, heartbeat timestamp
   - Atomic transaction prevents race conditions

2. Create scheduler lock utility (app/utils/scheduler_lock.py)
   - acquire_scheduler_lock(): Atomically acquire or fail
   - release_scheduler_lock(): Clean up on shutdown
   - update_scheduler_lock_heartbeat(): Keep lock alive (every 10 seconds)
   - get_scheduler_lock_info(): Debug/inspect lock status
   - Stale lock detection: TTL-based (60 second expiry)

3. Reorder startup DAG stages
   - DATABASE now comes first (required for lock acquisition)
   - WORKER_MODE depends on DATABASE (performs lock check after initialization)
   - Maintains all other stage dependencies intact

4. Update startup process (app/startup.py)
   - Replace _check_single_worker_mode() with two-tier check:
     * Fast check: BANGUI_WORKERS env var (if explicitly set to >1)
     * Authoritative check: Database lock (catches misconfiguration)
   - Return startup_db from startup_shared_resources() for lock management

5. Register scheduler lock heartbeat task
   - New task: scheduler_lock_heartbeat (app/tasks/scheduler_lock_heartbeat.py)
   - Updates lock heartbeat every 10 seconds (keeps lock alive)
   - Prevents false positives from temporary load spikes

6. Add lock release to lifespan shutdown (app/main.py)
   - Release lock before closing database
   - Allows other instances to acquire during rolling deployments
   - Graceful handoff between instances

7. Comprehensive test coverage (backend/tests/test_scheduler_lock.py)
   - Lock acquisition success and failure cases
   - Stale lock cleanup on startup
   - Lock release and heartbeat updates
   - Full lifecycle: acquire → heartbeat → release

8. Update documentation (Docs/Architekture.md § 9.3)
   - Explain single-executor requirement
   - Document database-backed locking mechanism
   - Compare with alternative approaches (filesystem, env var)
   - Include troubleshooting guide
   - Container orchestration examples (Docker, Kubernetes, systemd)

Why database-backed instead of filesystem?
   - Atomicity: SQLite transactions prevent TOCTOU race windows
   - Container-safe: Works across containers with shared DB volumes
   - No NFS/SMB edge cases
   - Timestamp-based stale detection (PID reuse is unreliable)
   - More reliable in rolling deployments

Benefits:
   - Works with any process manager (uvicorn, gunicorn, etc.)
   - Handles simultaneous startup attempts correctly
   - Automatic failover on instance crash (stale lock cleanup)
   - Clear error messages with troubleshooting steps
   - No environment variable required (lock is authoritative)
   - Scales to multi-worker deployments if combined with external job store

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/db.py

@@ -107,7 +107,7 @@ _SCHEMA_STATEMENTS: list[str] = [
     _CREATE_HISTORY_ARCHIVE,
 ]
 
-_CURRENT_SCHEMA_VERSION: int = 3
+_CURRENT_SCHEMA_VERSION: int = 4
 
 _MIGRATIONS: dict[int, str] = {
     1: "\n".join(_SCHEMA_STATEMENTS),
@@ -130,6 +130,19 @@ CREATE UNIQUE INDEX idx_sessions_token_hash ON sessions (token_hash);
 -- Tracks when each IP was last referenced to enable purging of stale entries.
 -- Default to current timestamp for existing rows.
 ALTER TABLE geo_cache ADD COLUMN last_seen TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'));
+""",
+    4: """
+-- Migration 4: Add scheduler_lock table for multi-worker safety.
+-- Implements database-backed locking to ensure only one worker runs the scheduler.
+-- Uses atomic transactions to prevent race conditions in container orchestration.
+-- Lock is held by the process that successfully inserts the singleton row (id=1).
+CREATE TABLE scheduler_lock (
+    id INTEGER PRIMARY KEY CHECK (id = 1),
+    pid INTEGER NOT NULL,
+    hostname TEXT NOT NULL,
+    created_at REAL NOT NULL,
+    heartbeat_at REAL NOT NULL
+);
 """,
 }