feat: manual Proxmox re-sync button + make connection config env-only

Add a 'Re-sync now' button to the Proxmox auto-sync settings section that
triggers an immediate inventory import (POST /proxmox/sync-now) using the
server env config — the manual counterpart to scheduled auto-sync.

Also fix a dual-source-of-truth bug: Proxmox connection config (host, port,
token, verify_tls) is now env-only and never persisted to scan_config.json.
Previously save_overrides() dumped host/port/verify alongside scan settings,
so saving an unrelated setting wrote an empty proxmox_host that load_overrides
then clobbered PROXMOX_HOST with on every boot. Only the auto-sync activation
(sync_enabled + sync_interval) stays user-editable and persisted.

ha-relevant: no

backend/app/schemas/proxmox.py

@@ -64,8 +64,11 @@ class ProxmoxImportPendingResponse(BaseModel):
 
 
 class ProxmoxConfig(BaseModel):
-    """Non-secret Proxmox connection + auto-sync config. Never carries a token —
-    ``token_configured`` reflects whether a server-side token is present."""
+    """Non-secret Proxmox connection + auto-sync config (GET response).
+
+    Connection fields (host/port/verify_tls) are env-only and read-only here —
+    surfaced for display. ``token_configured`` reflects whether a server-side
+    token is present. Never carries the token itself."""
 
     host: str = ""
     port: int = Field(8006, ge=1, le=65535)
@@ -73,3 +76,12 @@ class ProxmoxConfig(BaseModel):
     sync_enabled: bool = False
     sync_interval: int = Field(3600, ge=300)
     token_configured: bool = False
+
+
+class ProxmoxSyncConfig(BaseModel):
+    """User-editable auto-sync config (POST body). The ONLY persisted Proxmox
+    settings. Connection fields (host/port/token/verify_tls) are env-only and
+    are deliberately not accepted here."""
+
+    sync_enabled: bool = False
+    sync_interval: int = Field(3600, ge=300)