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/main.py

@@ -50,7 +50,6 @@ class JobPayload(BaseModel):
     node: str
     enabled: bool = True
     cron_schedule: str
-    proxmox_storage: str
     backup_mode: str
     compression: str
     retention_type: str
@@ -64,7 +63,6 @@ class PartialJobPayload(BaseModel):
     node: str | None = None
     enabled: bool | None = None
     cron_schedule: str | None = None
-    proxmox_storage: str | None = None
     backup_mode: str | None = None
     compression: str | None = None
     retention_type: str | None = None