feat: public template catalog + Vercel git integration

Backend: service_templates.is_public lets owners publish user templates to
the anonymous catalog (GET /api/v1/templates), mirroring Railway's
marketplace publish flow. Migration, sqlc queries, visibility rules, and
openapi.yaml updated; types regenerated.

App: template editor gains a "List in the public catalog" checkbox;
Public badge on catalog cards and detail view.

Site: new templates.html catalog page - search, category filters, Official
vs Community badges, detail modal with variables and copyable import JSON.
Fetches the live catalog from a reachable instance (?api= or localStorage
override) and falls back to templates.json, generated from SeedTemplates
via cmd/dumptemplates. Shared styles extracted to style.css.

Vercel: project connected to GitHub repo with rootDirectory=site;
.vercelignore limits CLI uploads to site/ so `vercel deploy --prod` from
repo root matches git builds.

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/migrations_goose/20260924120000_template_publishing.sql

@@ -0,0 +1,11 @@
+-- +goose Up
+-- Railway-style template publishing: owners can mark a user template public so
+-- it appears in the anonymous catalog (GET /api/v1/templates) and on the
+-- landing page. Official templates stay official; is_public only matters for
+-- user templates.
+ALTER TABLE public.service_templates ADD COLUMN IF NOT EXISTS is_public boolean NOT NULL DEFAULT false;
+CREATE INDEX IF NOT EXISTS idx_service_templates_public ON public.service_templates(is_public) WHERE is_public;
+
+-- +goose Down
+DROP INDEX IF EXISTS public.idx_service_templates_public;
+ALTER TABLE public.service_templates DROP COLUMN IF EXISTS is_public;