Integrate DDNS registration and IP update into installer

setup_cell.py: register_with_ddns() called at end of setup — detects
public IP via api.ipify.org, generates TOTP code from DDNS_TOTP_SECRET,
POSTs to DDNS /register, saves token to data/api/.ddns_token (mode 600).
Idempotent: skips if token file already exists. Fails gracefully if
DDNS_TOTP_SECRET is unset or network is unreachable.

scripts/ddns_update.py: standalone script for periodic IP updates.
Reads token from data/api/.ddns_token, fetches current public IP,
compares to cached last IP (data/api/.ddns_last_ip) and calls /update
only when the IP has actually changed.

Makefile: add ddns-update (run update script) and ddns-register (force
re-registration by removing old token then calling register_with_ddns).
Usage: DDNS_TOTP_SECRET=<secret> make ddns-register

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Makefile

@@ -12,7 +12,8 @@
         test-e2e-deps test-e2e-api test-e2e-ui test-e2e-wg test-e2e \
         reset-test-admin-pass \
         show-admin-password reset-admin-password \
-        show-routes add-peer list-peers
+        show-routes add-peer list-peers \
+        ddns-update ddns-register
 
 # Detect docker compose command (v2 plugin preferred, fallback to v1 standalone)
 DC := $(shell docker compose version >/dev/null 2>&1 && echo "docker compose" || echo "docker-compose")
@@ -335,6 +336,21 @@ add-peer:
 		echo "Usage: make add-peer PEER_NAME=name PEER_IP=10.0.0.x PEER_KEY=<pubkey>"; \
 	fi
 
+# ── DDNS ─────────────────────────────────────────────────────────────────────
+
+ddns-update:
+	@python3 scripts/ddns_update.py
+
+ddns-register:
+	@DDNS_TOTP_SECRET="$(DDNS_TOTP_SECRET)" python3 -c "\
+import os, sys; sys.path.insert(0, 'scripts'); \
+from setup_cell import register_with_ddns, _read_existing_ip_range; \
+import json; \
+cfg = json.load(open('config/api/cell_config.json')) if os.path.exists('config/api/cell_config.json') else {}; \
+name = cfg.get('_identity', {}).get('cell_name', os.environ.get('CELL_NAME', 'mycell')); \
+import os; os.remove('data/api/.ddns_token') if os.path.exists('data/api/.ddns_token') else None; \
+register_with_ddns(name)"
+
 # ── Dev ───────────────────────────────────────────────────────────────────────
 
 dev: