chore: release-prep docs, env vars, compose, and e2e port hygiene

- README rewritten end-to-end: stack, quick start, dev workflow, full
  /api/v1 endpoint table, error and pagination envelopes, auth
  quick-start (browser + bot bearer), configuration table, deployment
  notes, backup/restore pointer. Stale "next features" section dropped
  now that all eight feat branches are in.
- .env.example now lists every env var the backend reads, with
  inline explanations:
  - COOKIE_SECURE / COOKIE_DOMAIN / SESSION_TTL_DAYS (auth)
  - CORS_ALLOWED_ORIGINS (same-origin by default)
  - MAX_REQUEST_BYTES / MAX_FILE_BYTES (upload caps)
  - Postgres + storage + log vars carried over.
- docker-compose.yml forwards all of the above into the backend
  service with `${VAR:-default}` so an unset value falls back to the
  same default the code uses, and any `.env` override flows through
  without a compose edit.
- docs/backup.md: step-by-step backup, restore, and smoke-test drill
  for both stateful volumes (postgres-data + storage-data), plus a
  list of what's deliberately *not* in the backup (e.g., .env).
- playwright.config.ts: pins the e2e dev server to port 5174 with
  `--strictPort` so it neither reuses nor silently bumps off
  collision with another vite instance on 5173. Drops the flaky
  manual-start workflow the earlier branches needed.
- docker-compose syntax (both prod and dev) validates cleanly against
  .env.example with no undefined-variable warnings.

No version bump — this is documentation, config, and tooling.

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

frontend/playwright.config.ts

@@ -1,17 +1,25 @@
 import { defineConfig } from '@playwright/test';
 
+// Playwright runs the SvelteKit dev server on a dedicated port so it
+// doesn't reuse whatever the dev workflow has on the vite default
+// (`5173`). `--strictPort` makes vite fail fast on collision instead of
+// silently bumping to a free port, which would leave Playwright pointed
+// at the wrong host. Set `E2E_BASE_URL` to skip the embedded server and
+// point at an already-running deployment.
+const E2E_PORT = 5174;
+
 export default defineConfig({
     testDir: 'e2e',
     timeout: 30_000,
     use: {
-        baseURL: process.env.E2E_BASE_URL ?? 'http://localhost:5173',
+        baseURL: process.env.E2E_BASE_URL ?? `http://localhost:${E2E_PORT}`,
         trace: 'retain-on-failure'
     },
     webServer: process.env.E2E_BASE_URL
         ? undefined
         : {
-              command: 'npm run dev',
-              port: 5173,
+              command: `npm run dev -- --port ${E2E_PORT} --strictPort`,
+              port: E2E_PORT,
               reuseExistingServer: !process.env.CI,
               timeout: 120_000
           }