security(paste-image): harden against 7 findings from PR #84 review (#90)

Hardens `/api/sessions/:id/paste-image` against the seven findings flagged in the dismissed security review on #84. Each commit addresses one finding.

- LOW: Collision-free filenames (`paste-${ts}-${rand4}${ext}`)
- MED: Symlink check on image dir (`lstat` + non-recursive mkdir + `O_EXCL|O_NOFOLLOW`)
- MED: Magic-byte validation (PNG/JPEG/GIF/WebP/BMP)
- HIGH: CSRF protection (Origin/Referer match req.host; non-browser clients send `X-Codeman-CSRF`)
- MED: Swap hand-rolled multipart parser to @fastify/multipart with `limits: { fileSize: 10MB, files: 1, fields: 4 }`
- MED: Rate limit (30/min per IP+session) + hourly GC of `paste-*` files older than 7d
- LOW: Use `terminal.paste(text)` instead of `sendInput(text)` so bracketed-paste markers survive

Co-authored-by: Aamer Akhter <aakhter@gmail.com>

package.json

@@ -52,6 +52,7 @@
   "dependencies": {
     "@fastify/compress": "^8.3.1",
     "@fastify/cookie": "^11.0.2",
+    "@fastify/multipart": "^10.0.0",
     "@fastify/static": "^8.0.0",
     "@fastify/websocket": "^11.2.0",
     "@xterm/addon-fit": "^0.11.0",