feat(scheduler): admin run-now/run-all triggers + live progress + completion notices

Add per-job "Run now" buttons and a "Start all now" button to the admin Scheduler
dashboard (admin-gated endpoints). Triggers run the job in a background thread
independent of its interval, refusing a concurrent run (409). While running, the
long jobs (maintenance, enrich, backfill, shorts) report live progress through a
decoupled contextvar sink, shown as a progress bar on the job row via the existing
4s poll. A manually-triggered run posts a completion notification to the triggering
admin's inbox (scheduled runs stay silent to avoid spam); the inbox renders the
"scheduler" type trilingually from type+data. While here, give the maintenance job
its missing dashboard label/description in all three languages.

backend/app/progress.py

@@ -0,0 +1,40 @@
+"""Lightweight, decoupled progress reporting for long-running scheduler jobs.
+
+A job's work function calls `report(current, total, phase)`; the scheduler binds a sink (via
+`bind()`) that forwards it into the in-memory activity the admin dashboard polls. A contextvar
+holds the sink, so the sync code stays unaware of the scheduler and concurrent jobs (each in
+its own thread/context) never cross wires. When no sink is bound (e.g. a manual sync endpoint),
+`report` is a cheap no-op.
+"""
+import contextvars
+from typing import Callable, Optional
+
+Sink = Callable[[int, Optional[int], Optional[str]], None]
+
+_sink: contextvars.ContextVar[Optional[Sink]] = contextvars.ContextVar(
+    "progress_sink", default=None
+)
+
+
+def report(current: int, total: int | None = None, phase: str | None = None) -> None:
+    """Report job progress to the bound sink, if any. `total=None` means indeterminate."""
+    sink = _sink.get()
+    if sink is not None:
+        sink(current, total, phase)
+
+
+class bind:
+    """Context manager that binds a progress sink for the duration of a job run."""
+
+    def __init__(self, sink: Sink):
+        self._sink = sink
+        self._token = None
+
+    def __enter__(self) -> "bind":
+        self._token = _sink.set(self._sink)
+        return self
+
+    def __exit__(self, *exc) -> bool:
+        if self._token is not None:
+            _sink.reset(self._token)
+        return False