Make proxmox_storage a global setting instead of a per-job snapshot

Backup jobs stored their own proxmox_storage column, copied from
settings at creation time and never resynced. process_backup preferred
that frozen job value, so changing Settings > Proxmox storage had no
effect on existing jobs. Drop the per-job column and always use the
current global setting when starting a backup.

Also copies AGENTS.md to CLAUDE.md.

backend/app/migrations.py

@@ -179,3 +179,10 @@ def run_migrations() -> None:
                 "INSERT INTO schema_migrations(version, applied_at) VALUES (?, ?)",
                 (4, now),
             )
+        if 5 not in applied:
+            now = utc_now()
+            conn.execute("ALTER TABLE backup_jobs DROP COLUMN proxmox_storage")
+            conn.execute(
+                "INSERT INTO schema_migrations(version, applied_at) VALUES (?, ?)",
+                (5, now),
+            )