fix(deploy): unblock production bring-up (healthchecks + Caddy DOMAIN)

Two issues would each stop a clean production `docker compose up` for the event:

1. Healthchecks probed http://localhost:{3000,3001}, but the app and frontend
   bind IPv4 (0.0.0.0) while `localhost` resolves to ::1 (IPv6) first inside the
   container — so the probe got "connection refused" and neither container ever
   turned healthy. Caddy is gated on `condition: service_healthy` for both, so on
   a fresh boot it would block forever and nothing gets served. Switch both probes
   to 127.0.0.1. (Verified: both containers now report healthy.)

2. The prod caddy service never received DOMAIN, so the Caddyfile's `{$DOMAIN}`
   site address expanded to empty — malformed site block, no TLS, no serving. Add
   `environment: { DOMAIN: ${DOMAIN} }` to the caddy service.

Also make .env.example honest and event-ready:
- Add DATABASE_MAX_CONNECTIONS (real env lever; recommend 30 for ~100 guests).
- The DEFAULT_* upload/rate/capacity vars are NOT read from env — they are seeded
  into the DB config table and managed at runtime via the admin dashboard. Replace
  the misleading entries (e.g. upload rate showed 10; live value is 100 via
  migration 015) with a note pointing to the admin UI and the real seeded defaults.
- Document that raising COMPRESSION_WORKER_CONCURRENCY also needs the app memory
  limit raised (ffmpeg), so a video burst can't OOM the box.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -46,7 +46,11 @@ services:
     expose:
       - "3000"
     healthcheck:
-      test: ["CMD-SHELL", "wget -q -O- http://localhost:3000/health || exit 1"]
+      # Use 127.0.0.1, NOT localhost: the app binds IPv4 (0.0.0.0) but `localhost`
+      # resolves to ::1 (IPv6) first inside the container, so a localhost probe gets
+      # "connection refused" and the container never turns healthy — which would leave
+      # Caddy (gated on `condition: service_healthy` below) blocked forever on boot.
+      test: ["CMD-SHELL", "wget -q -O- http://127.0.0.1:3000/health || exit 1"]
       interval: 10s
       timeout: 5s
       retries: 5
@@ -73,7 +77,9 @@ services:
     expose:
       - "3001"
     healthcheck:
-      test: ["CMD-SHELL", "wget -q -O- http://localhost:3001/ >/dev/null 2>&1 || exit 1"]
+      # 127.0.0.1, not localhost — see the app healthcheck note above (IPv4 bind vs
+      # ::1 resolution would leave this container permanently unhealthy).
+      test: ["CMD-SHELL", "wget -q -O- http://127.0.0.1:3001/ >/dev/null 2>&1 || exit 1"]
       interval: 10s
       timeout: 5s
       retries: 5
@@ -86,6 +92,12 @@ services:
   caddy:
     image: caddy:2-alpine
     restart: unless-stopped
+    environment:
+      # The Caddyfile's site address is `{$DOMAIN}`, read from THIS container's env.
+      # Without it, `{$DOMAIN}` expands to empty, the site block collapses, and Caddy
+      # serves nothing / fails to obtain a TLS cert. `env_file` alone wouldn't help —
+      # Caddy needs it in `environment`, and this keeps the Caddyfile the single source.
+      DOMAIN: ${DOMAIN}
     ports:
       - "80:80"
       - "443:443"