feat(seo): Phase 2 — route-level metadata service and JSON-LD structured data

Add SeoService for dynamic metadata management:
- Per-route document titles (e.g., 'KMountain Flower Radio — About')
- Meta descriptions from station config per route
- Canonical URLs, Open Graph tags, Twitter Card tags
- Input sanitization (HTML stripping, URL validation, attribute escaping)
- noindex robots directives for /login and /admin routes
- Centralized router watcher (zero per-component boilerplate)

Add StructuredDataService + StructuredDataComponent for JSON-LD:
- Organization schema (NCBOrganization) site-wide with contact info
- WebSite schema with publisher info
- WebPage schema per-route with breadcrumbs
- Dynamic Event schema on /events page
- RadioSeason schema on /schedule page

Wire up SeoService.watchRouter() in app-root ngOnInit.
Wire up StructuredDataComponent in app-root template.
Wire up EventsComponent and ScheduleComponent to feed data to
StructuredDataService for dynamic schema injection.

Includes full test suite for SeoService (22 tests).

src/app/app.html

@@ -1,3 +1,4 @@
+<app-structured-data></app-structured-data>
 <app-navbar></app-navbar>
 <app-media-player></app-media-player>
 <main [class.no-stream]="!hasStream()">