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/api/status.py

@@ -112,8 +112,16 @@ async def get_nav_counts(
     user: User = Depends(get_current_user),
     session: AsyncSession = Depends(get_session),
 ):
-    """Return entity counts for sidebar navigation badges."""
-    counts = {}
+    """Return entity counts for sidebar navigation badges.
+
+    Note: queries run sequentially because SQLAlchemy AsyncSession is NOT safe
+    for concurrent use within a single session (no asyncio.gather).  We
+    minimise round-trips by combining user + system counts and per-type
+    target counts into single aggregate queries where possible.
+    """
+    counts: dict[str, int] = {}
+
+    # --- 1) User-owned entity counts (one query per model) ---
     for model, key in [
         (ServiceProvider, "providers"),
         (NotificationTracker, "notification_trackers"),
@@ -132,7 +140,7 @@ async def get_nav_counts(
         )).one()
         counts[key] = count
 
-    # System-owned entities (user_id=0) count as well
+    # --- 2) Add system-owned counts (user_id=0) for shared entities ---
     for model, key in [
         (TemplateConfig, "template_configs"),
         (CommandTemplateConfig, "command_template_configs"),
@@ -144,15 +152,22 @@ async def get_nav_counts(
         )).one()
         counts[key] += system_count
 
-    # Per-type target counts for nav badges
-    for target_type in ("telegram", "webhook", "email", "discord", "slack", "ntfy", "matrix"):
-        type_count = (await session.exec(
-            select(func.count()).select_from(NotificationTarget).where(
-                NotificationTarget.user_id == user.id,
-                NotificationTarget.type == target_type,
-            )
-        )).one()
-        counts[f"targets_{target_type}"] = type_count
+    # --- 3) Per-type target counts in a single query using conditional aggregation ---
+    target_types = ("telegram", "webhook", "email", "discord", "slack", "ntfy", "matrix")
+    type_counts_result = (await session.exec(
+        select(
+            NotificationTarget.type,
+            func.count(),
+        )
+        .where(
+            NotificationTarget.user_id == user.id,
+            NotificationTarget.type.in_(target_types),
+        )
+        .group_by(NotificationTarget.type)
+    )).all()
+    type_counts_map = dict(type_counts_result)
+    for target_type in target_types:
+        counts[f"targets_{target_type}"] = type_counts_map.get(target_type, 0)
 
     return counts