Fix preview worker pool leak: ffmpeg must not inherit worker stdin

The ffmpeg fallback in the preview worker inherited the worker's stdin
pipe (the framed request protocol). When a slow conversion was killed
at the 10s timeout, the orphaned ffmpeg grandchild kept that pipe open,
so the parent's proc.wait() blocked forever waiting for pipe EOF —
permanently sticking one dispatcher per event until the whole pool
starved and every preview request (pdf, image, office) returned 503.

- Run ffmpeg with stdin=DEVNULL (also stops it eating protocol bytes)
- Drop start_new_session (only needed for group kills, POSIX-only)
- Stop logging the master secret at worker startup

cista/preview.py

@@ -145,6 +145,9 @@ class _PreviewWorker:
 
     async def kill(self) -> None:
         if self.proc.returncode is None:
+            # Safe to hard-kill: the worker is stateless per request, and its
+            # subprocesses (ffmpeg) use stdin=DEVNULL so they never hold the
+            # worker's pipes open — proc.wait() cannot hang on pipe EOF.
             with contextlib.suppress(ProcessLookupError):
                 self.proc.kill()
             await self.proc.wait()
@@ -179,7 +182,6 @@ class _PreviewWorkerPool:
             stdin=asyncio.subprocess.PIPE,
             stdout=asyncio.subprocess.PIPE,
             stderr=asyncio.subprocess.PIPE,
-            start_new_session=True,
         )
         _active_procs.add(proc)
         try:

cista/preview_worker.py

@@ -219,7 +219,18 @@ def _image_via_ffmpeg(path: Path, maxsize: int, quality: int) -> bytes:
             cmd.insert(5, f"{new_w}x{new_h}")
     try:
         try:
-            subprocess.run(cmd, capture_output=True, check=True, shell=False)  # noqa: S603
+            # stdin=DEVNULL is critical: ffmpeg must not inherit the worker's
+            # stdin, which carries the framed request protocol. An inherited
+            # stdin lets ffmpeg eat protocol bytes and, if the worker is
+            # killed mid-conversion, keeps the orphaned ffmpeg holding the
+            # pipe open so the parent's proc.wait() hangs forever.
+            subprocess.run(  # noqa: S603
+                cmd,
+                capture_output=True,
+                check=True,
+                shell=False,
+                stdin=subprocess.DEVNULL,
+            )
         except subprocess.CalledProcessError as e:
             shell_cmd = shlex.join(cmd)
             stderr = (e.stderr or b"").decode(errors="replace").strip()
@@ -540,11 +551,7 @@ def main() -> None:
     logging.basicConfig(stream=sys.stderr, level=logging.INFO)
     try:
         config.load_config()
-        logger.warning(
-            "preview-worker config=%s master_secret=%s",
-            config.conffile,
-            config.config.secret,
-        )
+        logger.info("preview-worker config=%s", config.conffile)
     except Exception:
         logger.exception("preview-worker failed to load config at startup")
     if len(sys.argv) > 1: