feat: live frame progress streaming for cinematic renders

Replaced communicate() (blocking) with selectors-based line-by-line
stdout streaming — same pattern as still render. Each frame now
streams live to the frontend:

  [cinematic_render] Frame 42/480 -- 55.3s elapsed (0.76 fps)

Pipeline: Blender stdout → log_callback → emit() → Redis →
LiveRenderLog poll (2s) → frontend display

Also added log_callback parameter to cinematic render task call.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/render_blender.py

@@ -644,27 +644,47 @@ def render_cinematic_to_file(
     log_lines: list[str] = []
 
     t_render = time.monotonic()
+    import selectors as _sel
     proc = subprocess.Popen(
         cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE,
         text=True, env=env, start_new_session=True,
     )
-    try:
-        stdout, stderr = proc.communicate(timeout=7200)  # 2hr max for cinematic (480 frames)
-    except subprocess.TimeoutExpired:
-        try:
-            os.killpg(os.getpgid(proc.pid), signal.SIGTERM)
-        except (ProcessLookupError, OSError):
-            pass
-        stdout, stderr = proc.communicate()
+    stderr_lines: list[str] = []
+    deadline = time.monotonic() + 7200  # 2hr max
 
-    for line in (stdout or "").splitlines():
-        logger.info("[cinematic] %s", line)
-        if "[cinematic_render]" in line:
-            log_lines.append(line)
-            if log_callback:
-                log_callback(line)
-    for line in (stderr or "").splitlines():
-        logger.warning("[cinematic stderr] %s", line)
+    sel = _sel.DefaultSelector()
+    sel.register(proc.stdout, _sel.EVENT_READ, "stdout")
+    sel.register(proc.stderr, _sel.EVENT_READ, "stderr")
+
+    try:
+        while sel.get_map():
+            remaining = deadline - time.monotonic()
+            if remaining <= 0:
+                try:
+                    os.killpg(os.getpgid(proc.pid), signal.SIGTERM)
+                except (ProcessLookupError, OSError):
+                    pass
+                break
+            events = sel.select(timeout=min(remaining, 2.0))
+            for key, _ in events:
+                line = key.fileobj.readline()
+                if not line:
+                    sel.unregister(key.fileobj)
+                    continue
+                line = line.rstrip("\n")
+                if key.data == "stdout":
+                    logger.info("[cinematic] %s", line)
+                    if "[cinematic_render]" in line:
+                        log_lines.append(line)
+                        if log_callback:
+                            log_callback(line)
+                else:
+                    stderr_lines.append(line)
+                    logger.warning("[cinematic stderr] %s", line)
+    finally:
+        sel.close()
+
+    proc.wait()
 
     if proc.returncode != 0:
         raise RuntimeError(