feat: add POST /tabs/:tabId/set_input_files endpoint

[MrReasonable]

Why

AI agents using camofox-browser often need to apply for jobs or submit documents on behalf of users. Many job sites and portals use <input type="file"> elements for CV/resume uploads — but browsers intentionally block JavaScript from setting file inputs, so a standard type or click action cannot attach a file. This endpoint fills that gap by exposing Playwright's privileged setInputFiles channel via the REST API.

What

Adds POST /tabs/:tabId/set_input_files to programmatically attach files to <input type="file"> elements.

• Accepts either a snapshot ref (preferred, with automatic stale-ref refresh) or a CSS selector to target the file input
• File paths are sandboxed to a configurable uploads directory (CAMOFOX_UPLOADS_DIR, default /home/node/cv-uploads) to prevent the endpoint being used to read arbitrary files from the container
• Emits a tab:set_input_files plugin event consistent with the rest of the interaction surface
• Full OpenAPI/Swagger documentation included

Test plan

• Attaches file via CSS selector — filename appears in snapshot
• Attaches file via element ref — filename appears in snapshot
• Rejects paths outside the uploads directory with 400
• Rejects request with neither ref nor selector with 400
• Returns 404 for unknown tab ID
• Full e2e test suite passes (npm run test:e2e)

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