feat(photo-editor): NL editor planner, image ops, email/gallery server

Server-side photo editor: natural-language editor agent route
(/api/editor/agent), photo_editor tool family + style guide, image-op
provider proxy with progress, email server hardening, gallery/hwfit
routes, and graceful shutdown.

Source: vendor/photocraft (ad1cff44) editor-server files, merged onto
upstream/dev (a8c147b2). Conflict resolution documented in
pr_work/B_conflicts.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/shutdown.py

@@ -0,0 +1,34 @@
+"""Process-wide shutdown signalling for graceful restarts.
+
+Server-Sent-Events / streaming responses never complete on their own, so
+uvicorn's graceful shutdown would wait for an open browser stream up to the
+full ``timeout_graceful_shutdown`` window before exiting. A SIGTERM/SIGINT
+handler flips this flag and tells uvicorn to exit; every streaming generator
+checks the flag on each loop and ends promptly, so a restart completes in
+seconds while ordinary in-flight requests still finish.
+"""
+from __future__ import annotations
+
+import logging
+import threading
+
+logger = logging.getLogger(__name__)
+
+_SHUTTING_DOWN = threading.Event()
+
+
+def is_shutting_down() -> bool:
+    """True once a shutdown (SIGTERM/SIGINT) has been requested."""
+    return _SHUTTING_DOWN.is_set()
+
+
+def request_shutdown() -> None:
+    """Mark the process as shutting down (idempotent)."""
+    if not _SHUTTING_DOWN.is_set():
+        _SHUTTING_DOWN.set()
+        logger.info("shutdown requested; streaming responses will end promptly")
+
+
+def reset() -> None:
+    """Clear the flag (for tests)."""
+    _SHUTTING_DOWN.clear()