Add image upload support: assets dir, POST /api/assets, editor drop zone

CLAUDE.md

@@ -14,6 +14,7 @@ bincio_wiki/
   pages/          wiki content (*.md files the community edits)
     _docs/        software documentation (shown as separate section)
   blog/           blog/stories content (*.md files)
+  assets/         user-uploaded images (gitignored; rsync'd to VPS separately)
   site/           Astro 6 app (git submodule → brutsalvadi/astro-bloomz)
   edit/           FastAPI edit server (Python, port 8001)
   pyproject.toml  Python dependencies (managed by uv)
@@ -39,6 +40,11 @@ by `dev.sh --edit` — no manual setup needed. To add a dependency:
   without extension, e.g. `digital-garden`)
 - Docs section: `pages/_docs/*.md` (IDs start with `_docs/`)
 - Blog/stories: `blog/*.md`
+- Assets: `assets/` — gitignored, not part of the Astro build. FastAPI serves them
+  at `/assets/{filename}` in dev (Vite proxies `/assets` → port 8001). On the VPS,
+  nginx serves `/assets/` directly from the filesystem (rsync'd separately).
+  Upload via `POST /api/assets` (multipart); returns `{"url": "/assets/filename"}`.
+  The editor inserts `![name](/assets/filename)` into the markdown on upload.
 - Bonsai index: stored in `site/src/content/index/` — the `i.bonsai.md` file
   defines the semantic tree structure
 - The homepage (`site/src/pages/index.astro`) splits entries by `_docs/` prefix