feat: add --cprofile flag for performance profiling

Dumps cProfile stats to a file (default: s5p.prof) on exit.
View with: python -m pstats s5p.prof

.gitignore

@@ -7,3 +7,4 @@ dist/
 build/
 .venv/
 config/s5p.yaml
+*.prof

src/s5p/cli.py

@@ -42,6 +42,10 @@ def _parse_args(argv: list[str] | None = None) -> argparse.Namespace:
     )
     p.add_argument("-v", "--verbose", action="store_true", help="debug logging")
     p.add_argument("-q", "--quiet", action="store_true", help="errors only")
+    p.add_argument(
+        "--cprofile", metavar="FILE", nargs="?", const="s5p.prof",
+        help="enable cProfile, dump stats to FILE (default: s5p.prof)",
+    )
     return p.parse_args(argv)
 
 
@@ -73,7 +77,18 @@ def main(argv: list[str] | None = None) -> int:
     _setup_logging(config.log_level)
 
     try:
-        asyncio.run(serve(config))
+        if args.cprofile:
+            import cProfile
+            prof = cProfile.Profile()
+            prof.enable()
+            try:
+                asyncio.run(serve(config))
+            finally:
+                prof.disable()
+                prof.dump_stats(args.cprofile)
+                logging.getLogger("s5p").info("profile saved to %s", args.cprofile)
+        else:
+            asyncio.run(serve(config))
     except KeyboardInterrupt:
         return 0
     except Exception as e: