fix: resolve P0/P1 audit findings for security, architecture, and race conditions

See merge request !5

refactor(common): standardize hardcoded constants and enforce zero-hardcode architecture

See merge request !4

feat(core): implement essential saas and crawler lifecycle features

See merge request !3

fix(core): remediate backend audit findings across P0-P3 vulnerabilities and performance issues

See merge request !2

fix(core): resolve P0/P1 audit findings across security, tenant isolation, and scheduler'

See merge request !1

feat(backend): upgrade data crawler backend with schedule crawl, change detection, and Supabase support