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/watcher.py

@@ -6,7 +6,6 @@ import asyncio
 import logging
 from typing import Any
 
-import aiohttp
 from sqlmodel import select
 from sqlmodel.ext.asyncio.session import AsyncSession
 
@@ -102,19 +101,20 @@ async def check_tracker(tracker_id: int) -> dict[str, Any]:
 
     if provider_type == "immich":
         from notify_bridge_core.providers.immich import ImmichServiceProvider
-        async with aiohttp.ClientSession() as http_session:
-            immich = ImmichServiceProvider(
-                http_session,
-                provider_config.get("url", ""),
-                provider_config.get("api_key", ""),
-                provider_config.get("external_domain"),
-                provider_name,
-            )
-            connected = await immich.connect()
-            if not connected:
-                return {"status": "error", "reason": "failed to connect to provider"}
+        from .http_session import get_http_session
+        http_session = await get_http_session()
+        immich = ImmichServiceProvider(
+            http_session,
+            provider_config.get("url", ""),
+            provider_config.get("api_key", ""),
+            provider_config.get("external_domain"),
+            provider_name,
+        )
+        connected = await immich.connect()
+        if not connected:
+            return {"status": "error", "reason": "failed to connect to provider"}
 
-            events, new_state = await immich.poll(collection_ids, state_dict)
+        events, new_state = await immich.poll(collection_ids, state_dict)
     elif provider_type == "gitea":
         # Gitea is webhook-based — events arrive via /api/webhooks/gitea endpoint.
         # The scheduler still calls check_tracker but there's nothing to poll.
@@ -143,18 +143,22 @@ async def check_tracker(tracker_id: int) -> dict[str, Any]:
         events, new_state = await nut.poll(collection_ids, state_dict)
     elif provider_type == "google_photos":
         from notify_bridge_core.providers.google_photos import GooglePhotosServiceProvider
-        async with aiohttp.ClientSession() as http_session:
-            gp = GooglePhotosServiceProvider(
-                http_session,
-                provider_config.get("client_id", ""),
-                provider_config.get("client_secret", ""),
-                provider_config.get("refresh_token", ""),
-                provider_name,
-            )
-            connected = await gp.connect()
-            if not connected:
-                return {"status": "error", "reason": "failed to connect to Google Photos"}
-            events, new_state = await gp.poll(collection_ids, state_dict)
+        from .http_session import get_http_session
+        http_session = await get_http_session()
+        gp = GooglePhotosServiceProvider(
+            http_session,
+            provider_config.get("client_id", ""),
+            provider_config.get("client_secret", ""),
+            provider_config.get("refresh_token", ""),
+            provider_name,
+        )
+        connected = await gp.connect()
+        if not connected:
+            return {"status": "error", "reason": "failed to connect to Google Photos"}
+        events, new_state = await gp.poll(collection_ids, state_dict)
+    elif provider_type == "webhook":
+        # Webhook providers receive events via inbound HTTP; no polling needed.
+        return {"status": "ok", "events_detected": 0, "collections_checked": 0}
     else:
         return {"status": "error", "reason": f"unsupported provider type: {provider_type}"}