feat(seo): add metadata, sitemap, robots, JSON-LD, dynamic OG image

- Centralize brand/keyword config in lib/site.ts (slogan, 22 keywords).
- Root layout: metadataBase, title template, OG/Twitter defaults, robots rules.
- Per-page metadata: home, trips list (filter-aware), trip detail, profile (noindex).
- Layout wrappers add metadata to client-component pages: login, register, create-trip.
- Trip detail: generateMetadata + JSON-LD Event + BreadcrumbList (price, availability, rating).
- Home page: JSON-LD Organization + WebSite + SearchAction (sitelink search).
- app/sitemap.ts: dynamic sitemap pulling OPEN/FULL trips from Prisma.
- app/robots.ts: disallow /api/, /profile, /create-trip; references sitemap.
- app/trips/[id]/opengraph-image.tsx: dynamic 1200x630 OG image per trip with
  cover photo, title, mountain, date, price, brand badge.
- Seeder: switch trip images from local SVG placeholders to real Unsplash CDN URLs.
- Drop 18 obsolete seed SVGs from public/images/seed/.

New env: NEXT_PUBLIC_SITE_URL (defaults to localhost:3000) — set to prod domain on deploy.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/site.ts

@@ -0,0 +1,39 @@
+export const siteConfig = {
+  name: "SeTrip",
+  brand: "SeTrip",
+  slogan: "Masa cowok sejati, cewek seimut, nggak SeTrip bareng?",
+  description:
+    "SeTrip adalah platform open trip pendakian gunung yang menyatukan open trip dari berbagai sosial media ke satu tempat. Cari, gabung, dan mendaki bareng — dari pemula sampai pendaki rutin.",
+  keywords: [
+    "setrip",
+    "se trip",
+    "open trip",
+    "open trip gunung",
+    "open trip pendakian",
+    "open trip jawa barat",
+    "hiking",
+    "hiking bareng",
+    "mendaki bersama",
+    "naik gunung bareng",
+    "pendakian gunung",
+    "trip gunung",
+    "trip pendakian",
+    "gabung open trip",
+    "cari open trip",
+    "papandayan",
+    "ciremai",
+    "gunung gede",
+    "gunung pangrango",
+    "gunung guntur",
+    "gunung malabar",
+    "gunung tangkuban parahu",
+  ],
+} as const;
+
+export const siteUrl = (
+  process.env.NEXT_PUBLIC_SITE_URL ?? "http://localhost:3000"
+).replace(/\/$/, "");
+
+export function absoluteUrl(path = "/"): string {
+  return `${siteUrl}${path.startsWith("/") ? path : `/${path}`}`;
+}