fix: feishu block type detection, table rendering & image download

[dabuddha]

Summary

Fixes three critical issues in fetch_feishu.py that cause most document content to be lost or corrupted:

• Block type mapping is unreliable: The original code uses hardcoded block_type numbers (e.g., 10=Bullet, 17=Image) that don't match the actual API responses (where 12=Bullet, 27=Image). This causes bullet lists to render as empty code blocks and images to be completely lost.
• Tables are not supported: Table (block_type=31) and TableCell (block_type=32) blocks fall through to the else branch, producing garbled text instead of proper Markdown tables.
• Images are inaccessible: Images output as feishu-image://{token} — a custom protocol that no Markdown renderer can display.

Changes

1. detect_block_kind(block) — Detects block type by checking actual data keys ("image", "table", "bullet", etc.) instead of relying on block_type numbers. This is robust against API version differences.

2. render_table() + render_cell_content() — Renders Feishu Table blocks as proper Markdown tables, including images inside table cells.

3. download_image() + download_all_images() — Downloads document images via Feishu drive API (/drive/v1/medias/{token}/download) and saves them locally. Markdown references use relative local paths.

4. fetch_feishu_doc(save_dir=None) — New optional save_dir parameter. When provided, images are downloaded to {save_dir}/{title}_images/.

Backward Compatibility

• All new function parameters have default values — existing callers are unaffected
• --json mode behavior is unchanged (no image download)
• If image download fails (permission denied, network error), gracefully falls back to feishu-image:// protocol

Additional Permission Required

Image download requires the Feishu app to have drive:drive:readonly permission enabled. Without it, images will use the fallback feishu-image:// format (same as before).

Test plan

• Run python3 scripts/fetch_feishu.py <feishu_url> on a document with tables and images
• Verify tables render as Markdown table syntax (| col1 | col2 |)
• Verify images download to ~/Downloads/{title}_images/ directory
• Verify --json mode still works without downloading images
• Verify documents without tables/images still render correctly