feat(networks): add name field to networks

- Migration 0007: ALTER TABLE networks ADD COLUMN name TEXT NOT NULL DEFAULT ''
- Network model, repository, and API updated to include name
- Networks page: name input in the add form, Name column as first column in table
- Delete modal now shows "Name (CIDR)" for clarity
- Hosts page: network dropdowns now show network name instead of CIDR
- Seeds updated with names (LAN, DMZ, Corporate, VPN)

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

seeds/postgres/dev_seed.sql

@@ -7,15 +7,14 @@
 
 -- ── Networks ──────────────────────────────────────────────────────────────────
 
-INSERT INTO networks (cidr) VALUES
-    ('192.168.1.0/24'),
-    ('192.168.10.0/24'),
-    ('10.0.0.0/8'),
-    ('172.16.0.0/16')
+INSERT INTO networks (name, cidr) VALUES
+    ('LAN',       '192.168.1.0/24'),
+    ('DMZ',       '192.168.10.0/24'),
+    ('Corporate', '10.0.0.0/8'),
+    ('VPN',       '172.16.0.0/16')
 ON CONFLICT (cidr) DO NOTHING;
 
 -- ── Hosts ─────────────────────────────────────────────────────────────────────
--- Hosts have no UNIQUE constraint, so we guard with WHERE NOT EXISTS.
 
 -- LAN — 192.168.1.0/24
 INSERT INTO hosts (name, ip, network_id)