feat: comprehensive code review fixes — security, performance, quality

Backend security:
- Reject Gitea webhooks when webhook_secret is empty (was silently skipping)
- Add slowapi rate limiting on login (5/min) and setup (3/min) endpoints
- Add CORS middleware with configurable origins
- Mask telegram_webhook_secret in settings API response
- Protect system-owned command template configs from regular user modification
- Increase minimum password length to 8 characters

Backend performance:
- Batch queries in _resolve_command_context (3 queries instead of 3N)
- Concurrent album fetching with asyncio.gather in immich commands
- Singleton Jinja2 SandboxedEnvironment (reuse instead of per-render creation)
- TTLCache for rate limits (bounded memory, auto-eviction)
- Optional aiohttp session reuse in send_reply/send_media_group

Backend code quality:
- Extract dispatch_helpers.py (shared link_data loading + event filtering)
- Extract database/seeds.py from main.py (490 lines → dedicated module)
- Split immich_handler.py (415 lines) into commands/immich/ subpackage
- Replace bare except blocks with logged warnings
- Add per-provider config validation (Pydantic models)
- Truncate command input to 512 chars
- Expose usage_* and desc_* slots in capabilities and variables API

Frontend security:
- CSS.escape() for user-controlled querySelector in highlight.ts
- Client-side password min 8 chars validation on setup and password change

Frontend code quality:
- Replace any types with proper interfaces across top files
- Decompose targets/+page.svelte into TargetForm + ReceiverSection
- Fix $derived.by usage, $state mutation patterns
- Add console.warn to empty catch blocks

Frontend UX:
- Auth redirect via goto() with "Redirecting..." state
- Platform-aware Ctrl/Cmd K keyboard hint
- Remove stat-card hover transform

Frontend accessibility:
- Modal: role=dialog, aria-modal, focus trap, restore focus
- EntitySelect/IconGridSelect: listbox/option roles, aria-selected/disabled

frontend/src/lib/i18n/en.json

@@ -47,7 +47,7 @@
 		"createAccount": "Create account",
 		"creatingAccount": "Creating account...",
 		"passwordMismatch": "Passwords do not match",
-		"passwordTooShort": "Password must be at least 6 characters",
+		"passwordTooShort": "Password must be at least 8 characters",
 		"or": "or"
 	},
 	"dashboard": {
@@ -753,7 +753,8 @@
 		"filterByName": "Filter by name...",
 		"allTypes": "All types",
 		"allProviders": "All providers",
-		"noFilterResults": "No items match the current filter."
+		"noFilterResults": "No items match the current filter.",
+		"redirecting": "Redirecting..."
 	},
 	"gridDesc": {
 		"sortNone": "No sorting applied",