feat(api): extract pure API clients for image generation and TTS

- Add bin/api/ package with generate_image() and synthesize_tts()
- Refactor bin/commands/image.py to call bin.api.image.generate_image
- Refactor bin/commands/voice.py to call bin.api.voice.synthesize_tts
- API layer is free of Click CLI dependencies, usable by external scripts

bin/api/image.py

@@ -0,0 +1,57 @@
+"""Image generation API client (Doubao/Seedream)."""
+
+import base64
+import os
+import urllib.request
+from pathlib import Path
+
+_RATIO_MAP = {
+    "1:1": "1024x1024",
+    "4:3": "1024x768",
+    "3:4": "768x1024",
+    "16:9": "1280x720",
+    "9:16": "720x1280",
+}
+
+
+def generate_image(
+    api_key: str,
+    prompt: str,
+    size: str | None = None,
+    ratio: str | None = None,
+    output: str | None = None,
+    watermark: bool = True,
+    b64: bool = False,
+) -> str:
+    """Generate image using Doubao/Seedream API.
+
+    Returns absolute file path if saved, or URL string if output is None.
+    """
+    from openai import OpenAI
+
+    resolved_size = size or (ratio and _RATIO_MAP.get(ratio)) or "2K"
+
+    client = OpenAI(
+        base_url="https://ark.cn-beijing.volces.com/api/v3",
+        api_key=api_key,
+    )
+
+    resp = client.images.generate(
+        model="doubao-seedream-5-0-260128",
+        prompt=prompt,
+        size=resolved_size,
+        response_format="b64_json" if b64 else "url",
+        extra_body={"watermark": watermark},
+    )
+
+    if b64:
+        image_bytes = base64.b64decode(resp.data[0].b64_json)
+        save_path = output or "generated_image.png"
+        Path(save_path).write_bytes(image_bytes)
+        return os.path.abspath(save_path)
+    else:
+        url = resp.data[0].url
+        if output:
+            urllib.request.urlretrieve(url, output)
+            return os.path.abspath(output)
+        return url