feat: Add XTTS v2 support, refactor Docker/GPU infra, and improve Piper engine

- Add XTTS v2 configuration to .env.example
- Refactor Dockerfile to multi-stage build with CUDA 12.1 support
- Update Makefile with Kokoro and XTTS test environment targets
- Refactor Piper engine (app/engines/piper.py) to use python module execution
- Add comprehensive documentation for Kokoro and XTTS plans
- Add helper scripts and patches for build process

XTTS_IMPLEMENTATION_PLAN.md

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+# Coqui XTTS v2 Implementation Plan
+
+## Objective
+Implement Coqui XTTS v2 (`tts_models/multilingual/multi-dataset/xtts_v2`) as a new engine in AudioEngineHub.
+
+## 1. Dependencies (`requirements.txt`)
+- Add `TTS` (Coqui TTS).
+- **Note:** This is a heavy library. We will add it to `requirements.txt`.
+- **Potential Conflict:** `TTS` often requires specific `torch` versions. We need to ensure it plays nicely with `f5-tts` (which also uses torch) and `kokoro`.
+
+## 2. Configuration (`app/config.py` & `.env`)
+- `XTTS_DEVICE`: "cuda" or "cpu" (default: "cuda")
+- `XTTS_ACCEPT_LICENSE`: "true" (required to use the model)
+- `XTTS_MODEL_VERSION`: "v2.0.2" (or "main" for latest)
+
+## 3. Engine Implementation (`app/engines/xtts.py`)
+- **Class:** `XTTSEngine` (inherits `TTSEngineBase`)
+- **Init:**
+    - Load `TTS` API.
+    - Download/Load model: `tts_models/multilingual/multi-dataset/xtts_v2`.
+    - Handle license agreement.
+- **Synthesize:**
+    - Inputs: `text`, `speaker` (voice cloning reference), `language`.
+    - **Voice Cloning:** The `speaker` argument will be interpreted as a filename in `app/asset/voices/` (or a default provided sample).
+    - **Language:** XTTS supports 17 languages. We will map them (e.g., "en", "de", "fr").
+- **List Models:** Return `['xtts_v2']`.
+- **List Voices:** Scan `app/asset/voices/` for `.wav` files to use as reference speakers.
+
+## 4. System Updates
+- **`app/main.py`**: Register `xtts` in `ALL_ENGINES`.
+- **`Dockerfile`**: Ensure system dependencies (already have `ffmpeg` and `espeak-ng`, which are good).
+
+## 5. Directory Structure
+- `app/asset/voices/`: Directory to store reference audio files for cloning.
+
+## Action Plan
+1.  Update `requirements.txt`.
+2.  Create `app/engines/xtts.py`.
+3.  Update `app/config.py`.
+4.  Update `app/main.py`.