fix: add invoice_pricing_rules to project overview response and fix Decimal serialization

fix: convert Decimal to float in invoice pricing rules for JSON serialization

feat: add flexible invoice pricing system with tiered, flat rate, time-based, and custom pricing models

fix: correct ProjectRegion import path in projects API module

Enrich contractor invoice response with contractor/client/project names and public viewing link

Improve invoice generation by auto-deriving contractor/client from project and updating schemas, logic, and docs accordingly

Enhance available-tickets endpoint to auto-derive contractor from project and support contractor_id=auto

Fix contractor invoice list route by removing trailing slash to prevent 307 redirects and CORS failures

Fix contractor invoice route prefix conflict and update frontend docs to match corrected API paths

Add contractor invoice stats endpoint and provide focused frontend integration documentation