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/jobs.py

@@ -51,10 +51,10 @@ def create_job(payload: dict) -> dict:
             """
             INSERT INTO backup_jobs(
                 guest_vmid, guest_name, guest_type, node, enabled, cron_schedule,
-                proxmox_storage, backup_mode, compression, retention_type, retention_value,
+                backup_mode, compression, retention_type, retention_value,
                 next_run_at, created_at, updated_at
             )
-            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
             """,
             (
                 payload["guest_vmid"],
@@ -63,7 +63,6 @@ def create_job(payload: dict) -> dict:
                 payload["node"],
                 int(payload.get("enabled", True)),
                 payload["cron_schedule"],
-                payload["proxmox_storage"],
                 payload["backup_mode"],
                 payload["compression"],
                 payload["retention_type"],
@@ -88,7 +87,7 @@ def update_job(job_id: int, payload: dict) -> dict:
             """
             UPDATE backup_jobs SET
                 guest_vmid = ?, guest_name = ?, guest_type = ?, node = ?, enabled = ?,
-                cron_schedule = ?, proxmox_storage = ?, backup_mode = ?, compression = ?,
+                cron_schedule = ?, backup_mode = ?, compression = ?,
                 retention_type = ?, retention_value = ?, next_run_at = ?, updated_at = ?
             WHERE id = ?
             """,
@@ -99,7 +98,6 @@ def update_job(job_id: int, payload: dict) -> dict:
                 merged["node"],
                 int(merged["enabled"]),
                 merged["cron_schedule"],
-                merged["proxmox_storage"],
                 merged["backup_mode"],
                 merged["compression"],
                 merged["retention_type"],