[epub-tts] Vendor aedocw backends and add adapter subclasses

- Add git submodules under src/vendor for the aedocw repositories:
  - epub2tts, epub2tts-edge, epub2tts-chatterbox, epub2tts-kokoro
- Add adapter modules in src/epub_tts to run vendored packages in isolated venvs:
  - vendor_utils.py (creates/uses per-repo .venv and installs package)
  - epub2tts.py, epub2tts_edge.py, epub2tts_chatterbox.py, epub2tts_kokoro.py
- Add aedocw_backend.py providing AedocwBackend and concrete subclasses that
  dispatch to the vendored adapters when selected via the CLI.
- Add tts_edge shim for backward compatibility.
- Register new backends in src/epub_tts/__main__.py and invoke the selected
  backend at runtime.
- Update pyproject.toml to exclude vendored packages from setuptools package
  discovery.

This change vendors upstream tools while providing thin adapters that invoke
their CLIs inside isolated virtualenvs, and integrates them into the
epub-tts backend registry.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/epub_tts/__main__.py

@@ -7,13 +7,19 @@
 import sys
 from argparse import ArgumentParser
 
-from epub_tts import tts_backend
-from epub_tts import tts_edge
+from epub_tts import aedocw_backend, tts_backend, tts_edge
 
 backend = {
     # keys are the backend names, values are the corresponding TTS classes
     "default": tts_backend.GenericTTSBackend,
     "edge": tts_edge.TTSEdge,
+    # aedocw-backed implementations — these expect the corresponding
+    # package "console scripts"/entrypoints to be available in the
+    # environment (or an explicit backend_cmd to be provided).
+    "epub2tts": aedocw_backend.AedocwEpub2TTS,
+    "epub2tts-edge": aedocw_backend.AedocwEpub2TTSEdge,
+    "epub2tts-chatterbox": aedocw_backend.AedocwChatterbox,
+    "epub2tts-kokoro": aedocw_backend.AedocwKokoro,
 }
 
 
@@ -29,10 +35,13 @@ def main(args=None):
     p.add_argument("-o", "--output", help="Output audio file", required=True)
     p.add_argument("-l", "--language", help="Language to use for TTS")
     p.add_argument("-v", "--voice", help="Voice to use for TTS")
-    p.add_argument("-b", "--backend", 
-                   help="Backend to use for TTS",
-                   default="default",
-                   choices=backend.keys())
+    p.add_argument(
+        "-b",
+        "--backend",
+        help="Backend to use for TTS",
+        default="default",
+        choices=backend.keys(),
+    )
     args = p.parse_args(args or sys.argv[1:])
     print(
         f"Converting {args.input} to {args.output} using voice "
@@ -40,6 +49,14 @@ def main(args=None):
         f"backend {args.backend}, ..."
     )
 
+    backend_cls = backend[args.backend]
+    backend_instance = backend_cls(
+        backend_cmd=None,
+        language=args.language,
+        voice=args.voice,
+    )
+    backend_instance.run(args.input, args.output)
+
 
 if __name__ == "__main__":
     sys.exit(main())