fix(queue): claim a queue item before dispatch so it can't be reassigned mid-upload (#2615)

A queue row stays status='pending' for the whole FTP upload; status only flips
to 'printing' at the end. The edit routes only blocked non-pending rows, so a
PATCH during the upload window was accepted while the in-flight dispatch kept
using its snapshotted printer -- splitting the queue row from the archive /
expected-print / physical command across two printers, and enabling a duplicate
dispatch on restart. The #1853 CAS guards cancellation, not reassignment.

Add a dispatching_at claim, stamped atomically (WHERE status='pending' AND
dispatching_at IS NULL) before any slow I/O and cleared on every exit. While
held, the single-item PATCH returns 409 (re-checked just before the write),
bulk edits skip the row, and the scheduler won't re-select it. Startup
reconciliation clears claims orphaned by a crash mid-dispatch. The row stays
pending throughout, so no status/UI/completion/reconciliation path changes.

New column print_queue.dispatching_at (nullable, dialect-safe DDL). Covered by
scheduler tests (claim exclusivity, non-pending rejection, release-on-exit,
skip-already-claimed, startup stale-clear) and API tests (reassign 409,
printer_id unchanged, bulk skip, unclaimed row still edits).

backend/app/core/database.py

@@ -1521,6 +1521,22 @@ async def run_migrations(conn):
         except (OperationalError, ProgrammingError):
             pass  # Already applied
 
+    # Migration: Add dispatching_at claim column to print_queue (#2615). Nullable
+    # timestamp; the type differs by dialect (SQLite DATETIME vs Postgres
+    # TIMESTAMP) so an existing-DB upgrade doesn't hit "type datetime does not
+    # exist" on Postgres. On a fresh DB create_all() already built the column, so
+    # the ALTER is swallowed as "already exists".
+    #
+    # Placed AFTER the print_queue_new2 table-recreate above: that recreate
+    # (SQLite-only, and only on ancient DBs whose archive_id is still NOT NULL)
+    # rebuilds print_queue from an explicit column list that doesn't carry this
+    # column, so adding it earlier would let the recreate silently drop it. Adding
+    # it here means it survives that path.
+    if is_sqlite():
+        await _safe_execute(conn, "ALTER TABLE print_queue ADD COLUMN dispatching_at DATETIME")
+    else:
+        await _safe_execute(conn, "ALTER TABLE print_queue ADD COLUMN dispatching_at TIMESTAMP")
+
     # Migration: Add HA energy sensor entity columns to smart_plugs
     await _safe_execute(conn, "ALTER TABLE smart_plugs ADD COLUMN ha_power_entity VARCHAR(100)")
     await _safe_execute(conn, "ALTER TABLE smart_plugs ADD COLUMN ha_energy_today_entity VARCHAR(100)")