feat: GitHub Repo Marketplace (Bounty #857)

[508704820]

Builds a marketplace where developers can list their repos for bounty integration, browse available repos, and connect repo owners with bounty creators.

Listing System

RepoListing Model:

• repo_url, owner, name, description
• 10 categories: defi, nft, dao, infrastructure, developer_tools, ai_ml, gaming, social, security, other
• Stars, language, topics, bounty_compatible flag
• View count, install count tracking
• Rating system with user reviews

CRUD Operations:

• POST /api/marketplace/listings — Create (duplicate check)
• GET /api/marketplace/listings — List with filters (category, language, search, sort)
• GET /api/marketplace/listings/{id} — Get (auto-increment views)
• DELETE /api/marketplace/listings/{id} — Soft delete

5 Sort Modes:

• newest, stars, rating, bounties, popular (by views)

Search:

• Full-text across name, description, and topics
• Category and language filters
• Combinable filters

Rating System

• POST /api/marketplace/listings/{id}/rate
• 1-5 star scale with optional comment
• One rating per user (update on re-rate)
• Auto-calculated average rating

Stats API

• GET /api/marketplace/stats
• Total listings, views, installs
• Breakdown by category and language

Acceptance Criteria

• Browse and list repos for bounty integration (CRUD + search + filters + sort)
• Marketplace with categories and ratings (10 categories + 5-star rating system)
• Connect repo owners with bounty creators (bounty_compatible flag + bounty_count tracking)

Closes #857

Wallet: Lt9nERv6VHsojw15LpFeiaabuphAggzfLF9sM9UXRrZ