fix(t-paliad-077): /api/links/suggest 500 — switch to sqlx for paliad.link_*

The suggestion + feedback handlers wrote to legacy public-schema tables
(`patholo_link_suggestions`, `patholo_link_feedback`) via Supabase PostgREST.
The patHoLo→Paliad rebrand moved those tables into the paliad schema as
`paliad.link_suggestions` / `paliad.link_feedback` — PostgREST is not
configured to expose paliad on the youpc Supabase, so all three callsites
500'd in prod.

Replace the PostgREST integration with a new LinkService backed by the same
sqlx pool every other paliad service uses. Schema-qualified table names
work directly via DATABASE_URL, the inconsistent supabaseInsert/Count
helpers go away, and the suggestion/feedback handlers now use requireDB
for clean 503s when the pool isn't wired.

handleSuggestionCount keeps its tolerant 0-on-error behaviour so the admin
badge never blocks page render. When DATABASE_URL is unset the count
endpoint returns 0 instead of 503 — knowledge-platform-only deployments
still serve the Link Hub page.

Flagged in t-paliad-074 (F-12).

cmd/server/main.go

@@ -138,6 +138,7 @@ func main() {
 			Agenda:         services.NewAgendaService(pool, users),
 			Audit:          services.NewAuditService(pool),
 			EmailTemplate:  emailTemplateSvc,
+			Link:           services.NewLinkService(pool),
 		}
 		log.Println("Phase B services initialised")