refactor(phase3-session-b3): extract create, transform, compress, convert into sub-routers

Extracted remaining 4 endpoint groups:
- create.py: 7 endpoints (create, 3xtemplates, providers, estimate-cost, platform-specs)
- transform.py: 4 endpoints (image-to-video, talking-avatar, estimate-cost, video serving)
- compress.py: 5 endpoints (compress, batch, estimate, formats, presets)
- convert.py: 4 endpoints (convert-format, batch, supported, recommendations)

Legacy router is now empty (only imports + empty router definition).
All 33 routes preserved. Package is fully modular.

backend/routers/image_studio/__init__.py

@@ -1,6 +1,7 @@
 """Image Studio API router package.
 
 Composed from modular sub-routers. Same prefix and tags as the original monolithic file.
+Legacy router is kept as an empty anchor for backward compatibility.
 """
 
 from ..image_studio_router import router as legacy_router
@@ -10,6 +11,10 @@ from .control import router as control_router
 from .social import router as social_router
 from .edit import router as edit_router
 from .face_swap import router as face_swap_router
+from .create import router as create_router
+from .transform import router as transform_router
+from .compress import router as compress_router
+from .convert import router as convert_router
 
 legacy_router.include_router(health_router)
 legacy_router.include_router(upscale_router)
@@ -17,6 +22,10 @@ legacy_router.include_router(control_router)
 legacy_router.include_router(social_router)
 legacy_router.include_router(edit_router)
 legacy_router.include_router(face_swap_router)
+legacy_router.include_router(create_router)
+legacy_router.include_router(transform_router)
+legacy_router.include_router(compress_router)
+legacy_router.include_router(convert_router)
 
 router = legacy_router