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>

migrations/postgres/0007_add_network_name.sql

@@ -0,0 +1,3 @@
+-- Add a human-readable name to networks.
+-- DEFAULT '' allows the migration to run on databases that already have rows.
+ALTER TABLE networks ADD COLUMN name TEXT NOT NULL DEFAULT '';