fix(spoolbuddy): sync SSH key over heartbeat to survive Bambuddy keypair rotation

Bambuddy's SSH keypair under <DATA_DIR>/spoolbuddy/ssh/ regenerates whenever
  the data dir is recreated (volume remount, container recreate, fresh deploy).
  The daemon previously only fetched the pubkey at registration, so any
  rotation after a successful boot left ~/.ssh/authorized_keys pointing at
  a stale public half — every Update click then failed with "Connection
  closed by authenticating user spoolbuddy [preauth]" until the daemon was
  restarted by hand. Each prior registration also appended a fresh entry
  without pruning, accumulating stale Bambuddy-tagged keys indefinitely.

  - HeartbeatResponse now carries ssh_public_key; the heartbeat route reads
    it via the same try/except shape as the register route so a missing or
    unreadable backend key doesn't break telemetry.
  - _deploy_ssh_key() strips lines tagged bambuddy-spoolbuddy and writes
    the current key once. No-op when already in sync (no mtime churn on
    every heartbeat). User-managed entries are preserved.
  - Daemon heartbeat handler calls _deploy_ssh_key when the response
    carries a key, so rotations propagate within one heartbeat instead
    of requiring a service restart.

  Tests: 5 unit (creates-when-missing, replace-stale-pileup, preserve-user-keys,
  idempotent, swallows-write-errors) + 2 backend integration (heartbeat carries
  the key; backend key-read failure leaves ssh_public_key None but the
  heartbeat still 200s).

backend/app/api/routes/spoolbuddy.py

@@ -279,6 +279,17 @@ async def device_heartbeat(
     if was_offline:
         logger.info("SpoolBuddy device back online: %s", device.device_id)
 
+    # Include current SSH public key so the daemon can re-deploy it whenever
+    # Bambuddy's keypair rotates (data dir wiped, container recreated, etc.) —
+    # otherwise SSH updates fail until the daemon restarts.
+    ssh_public_key: str | None = None
+    try:
+        from backend.app.services.spoolbuddy_ssh import get_public_key
+
+        ssh_public_key = await get_public_key()
+    except Exception:
+        pass
+
     return HeartbeatResponse(
         pending_command=pending,
         pending_write_payload=pending_write,
@@ -287,6 +298,7 @@ async def device_heartbeat(
         calibration_factor=device.calibration_factor,
         display_brightness=device.display_brightness,
         display_blank_timeout=device.display_blank_timeout,
+        ssh_public_key=ssh_public_key,
     )