Add authenticated myAssigned bounty filter

[newmattock]

Summary

• require authentication when /gobounties/all?myAssigned=true is requested
• filter the existing bounty query by the authenticated user pubkey while preserving existing status/search/language/pagination filters
• add handler coverage for the assigned-to-me result and unauthenticated request path

Closes #2436
Bounty: https://community.sphinx.chat/bounty/3334

Companion PR

• Frontend support: Add Assigned to me bounty filter sphinx-tribes-frontend#1581

Testing

• go test ./handlers -run 'TestGetAllBounties' -count=1
• go test ./db -run '^$' -count=1
• gofmt -w db/db.go handlers/bounty.go handlers/bounty_test.go mocks/Database.go
• git diff --exit-code -- db/db.go handlers/bounty.go handlers/bounty_test.go mocks/Database.go
• git diff --check -- db/db.go handlers/bounty.go handlers/bounty_test.go mocks/Database.go