fix(audit-wave-10): build-auditor fixes — CSP, server externals, healthcheck

Address the highest-leverage CRITICAL/HIGH/MEDIUM items from the
build-auditor that weren't already covered by Wave 1 (EMAIL_REDIRECT_TO
production guard) or the existing `.dockerignore`.

**C3 — socket.io in standalone trace**
- Add socket.io + @socket.io/redis-adapter to serverExternalPackages
  in next.config so the build system sees the dependency (the custom
  server is the only importer, no Next route touches it).
- Belt-and-braces: COPY both from the deps stage into the runner stage
  of Dockerfile, mirroring the audit's suggested fix.

**H1 — CSP `'unsafe-inline'` in prod**
- Audit recommends nonce-based scripts. Implementing nonces requires
  middleware that emits a per-request nonce + threading it through
  Next's RSC bootstrap + Server Actions. Out of scope for this wave;
  documented the rationale at the CSP definition so the next pass
  knows where to start, and noted that the in-the-wild XSS surfaces
  are already closed via escapeHtml/escapeUrl in the email + webhook
  pipelines.

**H2 — NEXT_PUBLIC_APP_URL validation**
- Add `NEXT_PUBLIC_APP_URL: z.string().url()` to the env schema so a
  missing build-time value fails validation instead of silently
  inlining the empty string into the client bundle and breaking
  multi-origin deploys.

**M3 — serverExternalPackages completeness**
- Add imapflow, mailparser, pdf-lib, sharp, tesseract.js,
  @react-pdf/renderer, unpdf — all heavy native/CJS-leaning
  server-only deps that should not be route-traced.

**H5 — healthcheck PORT templatization**
- docker-compose.{,prod.}yml: replace hardcoded
  `http://localhost:3000/api/health` with `${PORT:-3000}` so
  overriding PORT via .env doesn't put the container into a
  restart loop.

**M9 — NODE_ENV=production in builder**
- Dockerfile builder stage now sets NODE_ENV=production above
  `RUN pnpm build` so the prod-only branches in next.config
  (CSP, etc.) compile deterministically.

**M7 — HEALTHCHECK directive in image**
- Add image-level HEALTHCHECK to the app Dockerfile (mirrors the
  one in Dockerfile.worker for Redis) so the image is
  self-describing for non-compose orchestrators.

Items already addressed prior to this wave:
- C1 (.dockerignore exists, comprehensive)
- C2 (EMAIL_REDIRECT_TO production refusal — Wave 1)
- H4 (compose resource + log limits — already in prod compose)

Tests 1315/1315 throughout.

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

src/lib/env.ts

@@ -69,6 +69,18 @@ const envSchema = z
     // App
     APP_URL: z.string().url(),
     PUBLIC_SITE_URL: z.string().url(),
+    /**
+     * Client-side bundle baseline URL. Inlined at build time by Next, so
+     * a missing value at build leaks into the browser as the empty
+     * string and forces fallbacks (`window.location.origin`) which
+     * silently work in dev and break on multi-origin deploys.
+     * build-auditor H2: validate at runtime so the bundle never ships
+     * with a blank baseline. The validation runs against
+     * `process.env.NEXT_PUBLIC_APP_URL` at build time; missing-at-build
+     * produces a clear validation error rather than a confusing
+     * runtime fallback.
+     */
+    NEXT_PUBLIC_APP_URL: z.string().url(),
     NODE_ENV: z.enum(['development', 'production', 'test']).default('development'),
     LOG_LEVEL: z.enum(['fatal', 'error', 'warn', 'info', 'debug', 'trace']).default('info'),
     /**