Remove open core licensing model

[mattwoberts]

Summary

• Removes the open core structure introduced in PR Content Moderation, revamped UI, and new "open core" licensing for some features. #1388, moves all code from commercial/ back into the main codebase
• Self-hosters now get all features (including content moderation) unconditionally
• For the hosted multi-tenant instance, pro features are gated behind isPro (based on subscription status)
• Removes the license key validation system entirely
• Renames HasCommercialFeatures to IsPro throughout backend and frontend
• Adds RequirePro middleware with route groups for clean pro feature gating
• Disables moderation at entity level when not pro, preventing orphaned moderation queues on downgrade

Changes

• Handlers: Moved commercial handler implementations into main codebase, removed handler registry pattern
• Services: Moved SQL implementations into main postgres service, removed license service
• Frontend: Moved components/pages from @commercial into standard paths, removed build aliases
• Routes: Pro features grouped under RequirePro() middleware
• Entity mapping: IsModerationEnabled forced to false when isPro is false

Test plan

• Verify go build compiles cleanly
• Verify golangci-lint run passes (0 issues)
• Run full test suite with make test
• Self-hosted: confirm all features including content moderation are available
• Hosted pro: confirm moderation works as expected
• Hosted free: confirm moderation page returns 403 and UI hides moderation controls
• Hosted downgrade scenario: confirm moderation is effectively disabled (no orphaned queue)