feat(game): serve panorama index from Neon Postgres

- panoramas + pano_provinces tables replace 28MB bundled JSON; pano-index.js
  now draws via cached COUNT + ORDER BY id OFFSET with rejection sampling,
  composite (province,id)/(district,id) indexes support the skip
- scripts/seed-pano-db.mjs validates pipeline artifacts (the old real-data
  vitest invariants, extracted to scripts/lib/pano-artifacts.mjs), stages
  into panoramas_next, verifies, renames into place in one transaction,
  keeps panoramas_old as backup; --province reseeds in place; --check
  validates only
- pipeline writes gitignored data-build/panos/; pano barrel removed
- tests run against PGlite mocked in at the @neondatabase/serverless
  boundary, mirroring the fake-upstash pattern; fixtures replace real data
- infrastructure errors rethrow instead of reading as missing coverage;
  session ids via crypto.randomUUID (uuid package dropped)

docs/tech-stack.md

@@ -1,7 +1,7 @@
 # Tech Stack
 
 ## Frontend Framework
-- **Next.js 15.5**: React-based full-stack framework with App Router
+- **Next.js 16**: React-based full-stack framework with App Router
 - **React 19.2**: Component architecture
 - **Tailwind CSS 4**: Utility-first CSS framework for styling
 
@@ -23,16 +23,23 @@
 ## Geographic Processing
 - **@turf/turf**: distance, point-in-polygon, union, simplify, and
   point-to-line distance -- used both at runtime and by the offline builds
-- **Generated region tree**: `src/data/regions/` holds the nodes,
-  `src/data/boundaries/<province>/` the simplified outlines, and
-  `src/data/panos/` the per-province panorama indexes
+- **Generated region tree**: `src/data/regions/` holds the nodes and
+  `src/data/boundaries/<province>/` the simplified outlines. The per-province
+  panorama indexes live in Postgres (see below), seeded from local pipeline
+  artifacts in `data-build/panos/` (gitignored)
 - **Client/server split**: `src/lib/regions.js` is the client-safe view and
-  imports nothing from `src/data/panos/` or `src/lib/pano-index.js`. That
+  imports nothing from `src/lib/pano-index.js` or `src/lib/pano-db.js`. That
   boundary is enforced by an import-graph walk in `tests/regions.test.js` -- the
-  indexes are ~29MB of exact answers
+  panorama rows are exact answers
 - **Server-side Calculations**: All geographic processing on backend
 
 ## Data Storage & Session Management
+- **Neon Postgres (HTTP)**: The panorama index -- 424k rows of id, province,
+  district, lat, lng -- read via `@neondatabase/serverless` (`DATABASE_URL` or
+  `POSTGRES_URL`). One random-draw query per round, count queries cached
+  per process. Seeded by `scripts/seed-pano-db.mjs`, which stages into
+  `panoramas_next`, verifies, then renames into place, keeping the previous
+  generation as `panoramas_old`
 - **Upstash Redis (REST)**: Session and leaderboard storage via `@upstash/redis` SDK (REST client, no sockets)
 - **Credential Flexibility**: Accepts `UPSTASH_REDIS_REST_URL`+`UPSTASH_REDIS_REST_TOKEN` (vanilla Upstash) or `KV_REST_API_URL`+`KV_REST_API_TOKEN` (Vercel Marketplace aliases)
 - **Multi-tenant Key Prefix**: All physical keys carry `KEY_PREFIX` (default `vngeoguessr:`) to safely share Upstash DB with other Vercel projects. Prefix applied centrally in `src/lib/upstash.js`; callers use logical keys only.
@@ -41,7 +48,7 @@
   `city:{regionCode}` for every province and district -- the `city:` segment is
   kept so existing scores stay addressable
 - **Sorted Sets**: Leaderboard ranking with automatic trimming (top 200)
-- **UUID v4**: Session identifier generation
+- **UUID v4**: Session identifier generation via built-in `crypto.randomUUID()`
 - **30-minute Session Expiry**: Automatic TTL-based cleanup
 
 ## UI Components & Styling
@@ -55,8 +62,14 @@
 ## Testing
 - **Vitest**: Test runner for the logic in `src/lib/`, the API routes, the
   generated region data, and the leaderboard migration
-- **In-memory Upstash fake**: Default backing store, no service required
+- **In-memory Upstash fake**: Default Redis backing store, no service required
+- **PGlite**: In-process Postgres (WASM) mocked in at the
+  `@neondatabase/serverless` boundary, so the panorama queries run against real
+  Postgres semantics with no service
 - **SRH + Redis (Docker)**: Optional lane running the same tests against real Redis
+- **Playwright**: Chromium smoke tests for the UI (`tests/e2e/`), with every
+  API call and the panorama image stubbed at the browser boundary -- no
+  services, no env vars
 
 ## Development & Analytics
 - **ESLint**: Code linting with Next.js configuration
@@ -65,7 +78,6 @@
 - **@vercel/speed-insights**: Performance monitoring
 
 ## Key Dependencies
-- **uuid**: unique session identifier generation
 - **@upstash/redis**: REST-based Upstash client; no socket pooling, works in edge compute and serverless
 - **JavaScript Only**: No TypeScript - pure JavaScript implementation
 - **Individual Parameters**: Functions use separate parameters instead of object destructuring