refactor: comprehensive codebase review — security, performance, quality, UX

Security:
- Fix NUT protocol command injection (validate names against safe regex)
- Enable Jinja2 autoescape=True to prevent HTML injection via external data
- Add WebhookProviderConfig validation model

Performance:
- Shared aiohttp.ClientSession singleton (replaces 40+ per-request sessions)
- Fix 4 N+1 queries with batch IN loads (poller, scheduler, memory, broadcast)
- asyncio.gather for Gitea commands and notification dispatcher
- Add DB indexes on NotificationTrackerState.tracker_id, CommandTrackerListener
- LRU cache for compiled Jinja2 templates
- Daily EventLog cleanup job (90-day retention)
- 30s HTTP timeout on all external calls
- GROUP BY for target type counts (replaces 7 sequential queries)

Code quality:
- Extract get_owned_entity() helper (replaces 11 duplicate functions)
- Extract slot_helpers.py (load_slots, save_slots, render_template_preview)
- Extract command_utils.py (tracker lookup, last event, collection IDs)
- Extract http_session.py (shared session lifecycle)
- Provider connection validation dedup (3x → 1 helper)
- Command dispatch tables replacing if/elif chains
- Album+links fetch helper (fetch_albums_with_links)
- Provider dispatch polymorphism (list_provider_collections)
- Immutable _enrich_assets (no longer mutates in-place)
- Fix _format_assets return type + handler unpacking

Frontend:
- Fix 18+ hardcoded English strings → t() with new i18n keys (en + ru)
- Mobile "More" nav panel with provider filter and search
- Shared Button.svelte component (4 variants, 2 sizes)
- Shared ErrorBanner.svelte component (8 pages updated)
- SvelteKit goto() replacing window.location.href
- Dashboard grid fixed for 4 cards, paginator opacity consistency

Functionality:
- max_instances=1 on scheduler jobs (prevents duplicate events)
- Webhook provider in watcher (prevents error spam)
- Fix stale SQLModel reference in poller
- Gitea get_repo() direct API call

packages/server/src/notify_bridge_server/services/http_session.py

@@ -0,0 +1,33 @@
+"""Application-level shared aiohttp.ClientSession.
+
+All outgoing HTTP requests in the server package should use the shared
+session returned by ``get_http_session()`` instead of creating
+per-request ``aiohttp.ClientSession`` instances.  This keeps a single
+TCP connection pool alive for the lifetime of the process, avoiding
+the overhead of pool creation/teardown on every request.
+
+Call ``close_http_session()`` once during application shutdown.
+"""
+
+from __future__ import annotations
+
+import aiohttp
+
+_DEFAULT_TIMEOUT = aiohttp.ClientTimeout(total=30)
+_session: aiohttp.ClientSession | None = None
+
+
+async def get_http_session() -> aiohttp.ClientSession:
+    """Get or create the shared HTTP session."""
+    global _session
+    if _session is None or _session.closed:
+        _session = aiohttp.ClientSession(timeout=_DEFAULT_TIMEOUT)
+    return _session
+
+
+async def close_http_session() -> None:
+    """Close the shared HTTP session (call on app shutdown)."""
+    global _session
+    if _session is not None and not _session.closed:
+        await _session.close()
+        _session = None