feat: Add Auto-Dubbing feature for Podcast Maker

This commit adds the Auto-Dubbing feature for Podcast Maker with support
for translating podcast audio to different languages with optional voice
cloning to preserve the original speaker's voice.

New Features:
- Translation Service (common module): DeepL integration for low-cost
  translation, WaveSpeed integration for high-quality translation
- Audio Dubbing Service: STT -> Translate -> TTS pipeline with
  voice cloning support
- 9 new API endpoints for dubbing and voice cloning
- Support for 34+ languages
- Cost estimation utilities
- Comprehensive documentation

Files Added:
- services/translation/ (5 files): Translation service module
- services/dubbing/: Audio dubbing service
- api/podcast/handlers/dubbing.py: API endpoints
- docs/AUTO_DUBBING.md: Feature documentation
- CHANGELOG.md: Change log

Files Modified:
- api/podcast/models.py: Added dubbing request/response models
- api/podcast/router.py: Added dubbing routes
- services/__init__.py: Export translation and dubbing services
- scene_animation.py: Fixed missing Path import

backend/CHANGELOG.md

@@ -0,0 +1,51 @@
+# Changelog
+
+All notable changes to the ALwrity project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## [Unreleased]
+
+### Added
+
+#### Auto-Dubbing Feature (Podcast Maker)
+- **Translation Service** (`backend/services/translation/`)
+  - Common translation module for use across the entire application
+  - DeepL integration for low-cost, high-quality text translation (500k chars/month free)
+  - WaveSpeed integration for high-quality video/audio translation
+  - Support for 34+ languages
+  - Batch translation support
+  - Factory pattern for provider selection
+  - Cost estimation utilities
+
+- **Audio Dubbing Service** (`backend/services/dubbing/`)
+  - Audio dubbing with STT → Translate → TTS pipeline
+  - Voice cloning support to preserve original speaker's voice
+  - Low-quality (DeepL) and high-quality (WaveSpeed) modes
+  - Batch dubbing support
+  - Cost estimation
+
+- **Podcast API Endpoints** (`backend/api/podcast/`)
+  - `POST /api/podcast/dub/audio` - Create audio dubbing task
+  - `GET /api/podcast/dub/{task_id}/result` - Get dubbing result
+  - `POST /api/podcast/dub/voices/clone` - Clone voice from audio sample
+  - `GET /api/podcast/dub/voices/{task_id}/result` - Get voice clone result
+  - `POST /api/podcast/dub/estimate` - Estimate dubbing cost
+  - `GET /api/podcast/dub/languages` - List supported languages
+  - `GET /api/podcast/dub/voices` - List available TTS voices
+
+- **Bug Fixes**
+  - Fixed missing `Path` import in `scene_animation.py`
+
+### Changed
+
+- Updated `backend/services/__init__.py` to export translation and dubbing services
+- Updated `.env` with DeepL API key placeholder
+
+### Documentation
+
+- Added `backend/docs/AUTO_DUBBING.md` with comprehensive feature documentation
+
+## [Previous Releases]
+
+See git history for previous changelog entries.