fix(ai): advance the progress bar during upscale and background removal (#608)

RealESRGAN's enhance() and rembg's remove() run in one opaque call, so the
progress bar froze at 30% for the whole inference. Add a time-based
heartbeat that advances the bar in a background thread while the model runs
and stops when it returns, so the bar moves instead of freezing. Verified
end to end on a GPU box (forced CPU): a 55s upscale emitted 26 steady ticks
then completed; background removal too.

Fixes #591

packages/ai/python/progress_heartbeat.py

@@ -0,0 +1,35 @@
+"""Slowly-rising progress heartbeat for opaque model calls.
+
+RealESRGAN and rembg run the model in a single call with no per-step
+callback, so the progress bar would otherwise sit frozen for the whole
+inference. run_with_heartbeat advances the bar in a background thread while
+the call runs, purely to show the job is still alive, and stops the moment
+the call returns (or raises). It never reaches ``end``; the caller emits the
+real completion value once the work is done.
+"""
+import threading
+
+
+def run_with_heartbeat(fn, emit, start, end, stage, interval=2.0):
+    """Run ``fn()`` while emitting rising progress via ``emit(pct, stage)``.
+
+    Advances one percent every ``interval`` seconds from ``start`` toward
+    ``end`` (never past ``end - 1``). Returns ``fn()``'s value and propagates
+    any exception unchanged.
+    """
+    stop = threading.Event()
+
+    def beat():
+        pct = start
+        while not stop.wait(interval):
+            if pct < end - 1:
+                pct += 1
+                emit(pct, stage)
+
+    thread = threading.Thread(target=beat, daemon=True)
+    thread.start()
+    try:
+        return fn()
+    finally:
+        stop.set()
+        thread.join(timeout=1.0)