feat(setup): Stage 1 first-time setup wizard backend (no UI)

Task #534 — backend, infra, tooling. UI ships in Stage 2 (#535).

Backend
- New system_settings table (id=1 singleton): installed flag, base_url,
  local_domain, local_ip, https_mode, app_version. Pushed to dev DB.
- New /api/setup/status (open) and /api/setup/{validate,complete}
  (gated by requireSetupOpen — 409 once installed).
- completeInstall is fully transactional: pg_advisory_xact_lock
  serializes concurrent callers, double-gates on installed flag and
  admin existence, then atomically creates the admin user, assigns
  admin role + Admins/Everyone groups, and flips system_settings to
  installed=true. Rolls back on any failure.
- Added redirectIfSetupNeeded() helper returning the full SetupStatus
  payload alongside a redirect target for SPA routing decisions.
- Zod validation, bcrypt hashing, in-memory rate limiter on the
  setup endpoints.

Backward compat
- scripts/src/seed.ts now branches on installed flag + admin
  existence + SEED_*_PASSWORD env vars. Legacy installs (admin
  exists, system_settings empty) get backfilled to installed=true
  via ON CONFLICT DO UPDATE so they are never forced through the
  wizard. When env passwords are unset and no admin exists, the
  seed prints a wizard hint instead of seeding.

Infra
- docker-compose.yml: replaced nginx edge with a Caddy service that
  mounts ./certs and ./docker/Caddyfile{,.skip}. The web service no
  longer publishes a port directly — Caddy is the only public ingress.
- Caddy entrypoint picks Caddyfile.skip (HTTP-only, no certs) when
  HTTPS_MODE=skip so a fresh host without mkcert can still boot.
- docker/Caddyfile: HTTPS site for LOCAL_DOMAIN/LOCAL_IP with
  WebSocket upgrade preserved and an HTTP→HTTPS redirect.
- start.sh: preserved. Now auto-picks HTTPS_MODE=skip when no cert
  is on disk and maps Caddy's HTTP_PORT to APP_PORT in skip mode so
  the legacy http://localhost:${APP_PORT} URL keeps working. In
  local/byo mode it prints the https://${LOCAL_DOMAIN} URL.
- .env.example: added LOCAL_DOMAIN, LOCAL_IP, BASE_URL, HTTP_PORT,
  HTTPS_PORT, HTTPS_MODE; SEED_*_PASSWORD now optional.

Tooling
- scripts/local-setup.sh: idempotent OS-aware bootstrap (.env upsert,
  mkcert hint, cert SAN check, dry-run via LOCAL_SETUP_DRY_RUN).

Tests
- artifacts/api-server/tests/setup-wizard.test.mjs: 7/7 pass.
- scripts/tests/local-setup.test.mjs: 2/2 pass.

Constraints honored: no force-push, no destructive ops, start.sh
preserved & still works, scripts idempotent, volumes/DB never
touched, HTTPS skip mode dev-only, wizard does not edit
LOCAL_DOMAIN/LOCAL_IP.

Out of scope / not addressed: pre-existing TS errors in
routes/users.ts and pre-existing failure in
executive-meetings-postpone-race.test.mjs.

artifacts/api-server/src/lib/setupService.ts

@@ -253,18 +253,31 @@ export async function completeInstall(
       return { ok: false, status: 409, error: "already_installed" };
     }
 
-    // Username/email uniqueness re-check inside the txn.
+    // Username/email uniqueness re-check inside the txn. We map
-    const dup = await client.query<{ id: number }>(
+    // collisions back to per-field keys so the wizard can highlight
-      `SELECT id FROM users WHERE username = $1 OR email = $2 LIMIT 1`,
+    // the offending input precisely.
+    const dup = await client.query<{
+      id: number;
+      username: string;
+      email: string;
+    }>(
+      `SELECT id, username, email FROM users WHERE username = $1 OR email = $2`,
       [input.admin.username, input.admin.email],
     );
     if ((dup.rowCount ?? 0) > 0) {
       await client.query("ROLLBACK");
+      const errors: ValidationErrors = {};
+      for (const row of dup.rows) {
+        if (row.username === input.admin.username)
+          errors["admin.username"] = "username_taken";
+        if (row.email === input.admin.email)
+          errors["admin.email"] = "email_taken";
+      }
       return {
         ok: false,
         status: 400,
         error: "validation_failed",
-        errors: { "admin.username": "taken_or_email_taken" },
+        errors,
       };
     }