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>

app/sitemap.ts

@@ -0,0 +1,43 @@
+import type { MetadataRoute } from "next";
+import { prisma } from "@/lib/prisma";
+import { absoluteUrl } from "@/lib/site";
+
+export default async function sitemap(): Promise<MetadataRoute.Sitemap> {
+  const trips = await prisma.trip.findMany({
+    where: { status: { in: ["OPEN", "FULL"] } },
+    select: { id: true, updatedAt: true },
+    orderBy: { updatedAt: "desc" },
+  });
+
+  const now = new Date();
+
+  const staticEntries: MetadataRoute.Sitemap = [
+    {
+      url: absoluteUrl("/"),
+      lastModified: now,
+      changeFrequency: "daily",
+      priority: 1,
+    },
+    {
+      url: absoluteUrl("/trips"),
+      lastModified: now,
+      changeFrequency: "hourly",
+      priority: 0.9,
+    },
+    {
+      url: absoluteUrl("/register"),
+      lastModified: now,
+      changeFrequency: "yearly",
+      priority: 0.3,
+    },
+  ];
+
+  const tripEntries: MetadataRoute.Sitemap = trips.map((t) => ({
+    url: absoluteUrl(`/trips/${t.id}`),
+    lastModified: t.updatedAt,
+    changeFrequency: "daily",
+    priority: 0.8,
+  }));
+
+  return [...staticEntries, ...tripEntries];
+}