Fix Phase 5 review issues: SSTI, FK violation, sync rebuild

Fixes 5 issues identified by code-reviewer agent:

1. (Critical) EventLog.tracker_id now nullable - use None instead
   of 0 when tracker name doesn't match, avoiding FK constraint
   violations on PostgreSQL
2. (Critical) Replace jinja2.Environment with SandboxedEnvironment
   in all 3 server template rendering locations to prevent SSTI
3. (Important) Rebuild sync_client in _async_update_listener when
   server URL/key options change, propagate to all coordinators
4. (Important) Validate partial server config - require both URL
   and API key or neither, with clear error message
5. (Important) Name fire-and-forget sync task for debugging

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

packages/server/src/immich_watcher_server/services/notifier.py

@@ -24,7 +24,7 @@ DEFAULT_TEMPLATE = (
 
 def render_template(template_body: str, context: dict[str, Any]) -> str:
     """Render a Jinja2 template with the given context."""
-    env = jinja2.Environment(autoescape=False)
+    env = jinja2.sandbox.SandboxedEnvironment(autoescape=False)
     tmpl = env.from_string(template_body)
     return tmpl.render(**context)