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.

docker/Caddyfile

@@ -1,12 +1,12 @@
-# Tx OS — Caddy reverse proxy + TLS termination.
+# Tx OS — Caddy reverse proxy + TLS termination (HTTPS_MODE=local|byo).
 #
-# Defaults (HTTPS_MODE=local or byo): listen on :443 with the mounted
-# certificate pair from /certs, redirect HTTP→HTTPS automatically.
-# When HTTPS_MODE=skip, we serve plaintext on :80 only (developer-only;
-# the host script never enables this in production paths).
+# This Caddyfile is the default. The compose `caddy` service entrypoint
+# loads docker/Caddyfile.skip instead when HTTPS_MODE=skip, so this file
+# can assume TLS certificates exist in /certs.
 #
 # The site MUST stay single-origin so the API session cookie
 # (sameSite=lax) keeps working across SPA + /api requests.
+
 {
 	# Disable Caddy's automatic Let's Encrypt issuance — we always
 	# bring our own cert (mkcert locally, real cert in BYO mode).
@@ -47,16 +47,9 @@
 	import spa_proxy
 }
 
-# Plain HTTP — issue a permanent redirect to HTTPS in normal modes,
-# but become the actual site when HTTPS_MODE=skip.
+# Plain HTTP → permanent redirect to HTTPS. Skip-mode (no certs) is
+# handled exclusively by docker/Caddyfile.skip, picked by the compose
+# entrypoint when HTTPS_MODE=skip.
 :80 {
-	@skip expression {$HTTPS_MODE:local} == "skip"
-	handle @skip {
-		encode zstd gzip
-		import api_proxy
-		import spa_proxy
-	}
-	handle {
-		redir https://{host}{uri} permanent
-	}
+	redir https://{host}{uri} permanent
 }