feat: add proprietary plugin upload support

[Rezn1r]

Description

Adds an alternative plugin creation flow for proprietary plugins that allows direct file upload of pre-built artifacts (.whl for Python, .so/.dll for C++) instead of connecting a GitHub repository. The build pipeline is bypassed — artifacts go directly to storage and still go through VirusTotal scanning and admin review.

Changes

• Schema: Add isProprietary Boolean @default(false) to Plugin model
• Middleware: Add requireTrusted (rejects NEW/FLAGGED users) and proprietaryUploadRateLimit (3/hour per user)
• Upload module (src/modules/upload/):
  • POST /api/v1/upload/plugin — create proprietary plugin with artifact upload
  • POST /api/v1/upload/plugin/:slug/version — upload new version for existing proprietary plugin
  • Magic number verification (.whl→ZIP, .so→ELF, .dll→MZ), filename sanitization, atomic failure cleanup
  • Immediate VT scan enqueue on upload, per-user plugin count limit (max 10)
• Submit service: Skip GitHub username validation for proprietary plugins, force "Proprietary" license
• Plugin service: Specific error message when attempting triggerBuild on proprietary plugins

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings