Add member-node association support

Members can now have multiple associated nodes, each with a public_key
and node_role (e.g., 'chat', 'repeater'). This replaces the single
public_key field on members with a one-to-many relationship.

Changes:
- Add MemberNode model for member-node associations
- Update Member model to remove public_key, add nodes relationship
- Update Pydantic schemas with MemberNodeCreate/MemberNodeRead
- Update member_import.py to handle nodes list in seed files
- Update API routes to handle nodes in create/update/read operations
- Add Alembic migration to create member_nodes table and migrate data
- Update example seed file with new format

example/seed/members.yaml

@@ -1,6 +1,16 @@
 # Example members seed file
+# Each member can have multiple nodes with different roles (chat, repeater, etc.)
 members:
   - name: Example Member
     callsign: N0CALL
     role: Network Operator
-    description: Example member entry
+    description: Example member entry with multiple nodes
+    nodes:
+      - public_key: 0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef
+        node_role: chat
+      - public_key: fedcba9876543210fedcba9876543210fedcba9876543210fedcba9876543210
+        node_role: repeater
+  - name: Simple Member
+    callsign: N0CALL2
+    role: Observer
+    description: Member without any nodes