feat(turn): self-hosted coturn support + time-limited creds + failure UX

- /api/ice: when TURN_SECRET is set, mint short-lived HMAC credentials
  (coturn use-auth-secret) so no permanent password is exposed and the relay
  can't be abused. Static TURN_USERNAME/CREDENTIAL still supported.
- share.html: connection watchdog + clear "couldn't connect on this network"
  message instead of a blank screen when no path can be established.
- deploy/coturn: ready-to-run turnserver.conf + docker-compose + README for
  hosting our own TURN on a VM we own (flat cost, no per-GB billing).

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

deploy/coturn/README.md

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+# Self-hosted TURN (coturn) for BizGaze Connect
+
+Why: customers behind symmetric NAT / corporate firewalls / VPNs can't form a direct
+WebRTC path, so screen share blanks out and disconnects. A TURN relay fixes it. We host
+our own coturn on a VM we already own — flat cost, no per-GB billing.
+
+## 1. VM prerequisites
+- A VM with a **public IP** (your data-center VM is fine).
+- A DNS A record, e.g. `turn.yourdomain.com` -> that public IP.
+- A TLS cert for that name (Let's Encrypt): `certbot certonly --standalone -d turn.yourdomain.com`
+
+## 2. Open firewall ports (on the VM and any edge firewall)
+- `3478/udp` and `3478/tcp`  (STUN/TURN)
+- `5349/tcp`                 (TURN over TLS)  — and `443/tcp` if you enable alt-tls
+- `49152-65535/udp`          (relay range)
+
+## 3. Configure
+Edit `turnserver.conf`:
+- `external-ip=` your VM's public IP
+- `static-auth-secret=` a long random string (e.g. `openssl rand -hex 32`)
+- `realm=` your domain
+- `cert=` / `pkey=` paths to your Let's Encrypt cert
+
+## 4. Run
+```
+docker compose up -d        # uses docker-compose.yml here
+# or native: apt install coturn; copy this file to /etc/turnserver.conf; enable in /etc/default/coturn; systemctl enable --now coturn
+```
+
+## 5. Point the app at it (production env)
+```
+TURN_URLS=turn:turn.yourdomain.com:3478,turn:turn.yourdomain.com:3478?transport=tcp,turns:turn.yourdomain.com:5349?transport=tcp
+TURN_SECRET=<the same static-auth-secret from turnserver.conf>
+TURN_TTL=86400
+```
+The app's `/api/ice` mints short-lived credentials from `TURN_SECRET` automatically — no
+permanent password is exposed, and outsiders can't reuse your relay. Restart the app.
+
+## 6. Verify
+- `GET https://<app>/api/ice` should return a `turn:`/`turns:` entry with a username + credential.
+- Test page: https://webrtc.github.io/samples/src/content/peerconnection/trickle-ice/
+  Add your `turns:turn.yourdomain.com:5349?transport=tcp` with the username/credential from
+  `/api/ice`; you should see a candidate of type **relay**. If you do, restrictive networks
+  are covered.