feat: replace hardcoded docker-compose IPs with .env-based substitution

docker-compose.yml now uses ${VAR:-default} for every container IP and
the network subnet, so there are no hardcoded addresses in the YAML.

How it works:
- setup_cell.py generates .env at project root from ip_range (gitignored).
- docker-compose reads .env automatically at startup.
- When ip_range changes in Settings, the API writes a new .env via
  ip_utils.write_env_file(); DNS/firewall/vIPs update immediately.
- User runs `make start` to recreate containers with the new IPs.

api/ip_utils.py gains ENV_VAR_NAMES dict and write_env_file(ip_range, path).
The old update_docker_compose_ips() direct-patch approach is removed from app.py.
3 new tests added (TestWriteEnvFile); total 324 pass.

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

tests/test_ip_utils.py

@@ -80,72 +80,67 @@ class TestGetVirtualIps(unittest.TestCase):
         self.assertEqual(vips['webdav'],   '10.10.0.24')
 
 
-class TestUpdateDockerComposeIps(unittest.TestCase):
-    COMPOSE_TEMPLATE = """\
-version: '3.3'
-services:
-  caddy:
-    networks:
-      cell-network:
-        ipv4_address: 172.20.0.2
-  dns:
-    networks:
-      cell-network:
-        ipv4_address: 172.20.0.3
-  api:
-    networks:
-      cell-network:
-        ipv4_address: 172.20.0.10
-networks:
-  cell-network:
-    ipam:
-      config:
-        - subnet: 172.20.0.0/16
-"""
-
+class TestWriteEnvFile(unittest.TestCase):
     def setUp(self):
-        self.tmp = tempfile.NamedTemporaryFile(mode='w', suffix='.yml', delete=False)
-        self.tmp.write(self.COMPOSE_TEMPLATE)
-        self.tmp.close()
+        self.tmp = tempfile.mkdtemp()
+        self.env_path = os.path.join(self.tmp, '.env')
 
     def tearDown(self):
-        os.unlink(self.tmp.name)
+        import shutil
+        shutil.rmtree(self.tmp)
 
-    def test_returns_false_for_missing_file(self):
-        self.assertFalse(
-            ip_utils.update_docker_compose_ips('172.20.0.0/16', '10.0.0.0/24', '/nonexistent/path.yml')
-        )
-
-    def test_subnet_updated(self):
-        ip_utils.update_docker_compose_ips('172.20.0.0/16', '10.0.0.0/24', self.tmp.name)
-        with open(self.tmp.name) as f:
-            content = f.read()
-        self.assertIn('10.0.0.0/24', content)
-        self.assertNotIn('172.20.0.0/16', content)
-
-    def test_caddy_ip_updated(self):
-        ip_utils.update_docker_compose_ips('172.20.0.0/16', '10.0.0.0/24', self.tmp.name)
-        with open(self.tmp.name) as f:
-            content = f.read()
-        self.assertIn('10.0.0.2', content)
-        self.assertNotIn('172.20.0.2', content)
-
-    def test_api_ip_updated(self):
-        ip_utils.update_docker_compose_ips('172.20.0.0/16', '10.0.0.0/24', self.tmp.name)
-        with open(self.tmp.name) as f:
-            content = f.read()
-        self.assertIn('10.0.0.10', content)
-        self.assertNotIn('172.20.0.10', content)
+    def test_creates_file(self):
+        ip_utils.write_env_file('172.20.0.0/16', self.env_path)
+        self.assertTrue(os.path.exists(self.env_path))
 
     def test_returns_true_on_success(self):
-        result = ip_utils.update_docker_compose_ips('172.20.0.0/16', '10.0.0.0/24', self.tmp.name)
+        result = ip_utils.write_env_file('172.20.0.0/16', self.env_path)
         self.assertTrue(result)
 
-    def test_noop_when_ranges_same(self):
-        ip_utils.update_docker_compose_ips('172.20.0.0/16', '172.20.0.0/16', self.tmp.name)
-        with open(self.tmp.name) as f:
-            content = f.read()
-        self.assertEqual(content, self.COMPOSE_TEMPLATE)
+    def test_returns_false_on_unwritable_path(self):
+        result = ip_utils.write_env_file('172.20.0.0/16', '/nonexistent/deep/path/.env')
+        self.assertFalse(result)
+
+    def test_contains_cell_network(self):
+        ip_utils.write_env_file('172.20.0.0/16', self.env_path)
+        content = open(self.env_path).read()
+        self.assertIn('CELL_NETWORK=172.20.0.0/16', content)
+
+    def test_contains_caddy_ip(self):
+        ip_utils.write_env_file('172.20.0.0/16', self.env_path)
+        content = open(self.env_path).read()
+        self.assertIn('CADDY_IP=172.20.0.2', content)
+
+    def test_contains_all_env_vars(self):
+        ip_utils.write_env_file('172.20.0.0/16', self.env_path)
+        content = open(self.env_path).read()
+        for var in ip_utils.ENV_VAR_NAMES.values():
+            self.assertIn(var + '=', content, f'{var} missing from .env')
+
+    def test_custom_subnet_generates_correct_ips(self):
+        ip_utils.write_env_file('10.5.0.0/24', self.env_path)
+        content = open(self.env_path).read()
+        self.assertIn('CELL_NETWORK=10.5.0.0/24', content)
+        self.assertIn('CADDY_IP=10.5.0.2', content)
+        self.assertIn('DNS_IP=10.5.0.3', content)
+        self.assertIn('API_IP=10.5.0.10', content)
+        self.assertNotIn('172.20', content)
+
+    def test_overwrite_updates_ips(self):
+        ip_utils.write_env_file('172.20.0.0/16', self.env_path)
+        ip_utils.write_env_file('10.0.0.0/24', self.env_path)
+        content = open(self.env_path).read()
+        self.assertIn('CADDY_IP=10.0.0.2', content)
+        self.assertNotIn('172.20', content)
+
+    def test_each_line_is_key_equals_value(self):
+        ip_utils.write_env_file('172.20.0.0/16', self.env_path)
+        for line in open(self.env_path).read().splitlines():
+            if line.strip():
+                self.assertIn('=', line, f'Bad line format: {line!r}')
+                key, _, val = line.partition('=')
+                self.assertTrue(key.isupper() or '_' in key)
+                self.assertTrue(val.strip())
 
 
 if __name__ == '__main__':