[EPAC-2222]: Decouple lobbying repository Queryer from pgx types

[riddim-developer-bot]

Summary

• Removed pgx-specific return types from the lobbying repository Queryer contract by replacing pgconn.CommandTag, pgx.Rows, and pgx.Row dependencies with repository-local database-agnostic interfaces.
• Updated repository scan helpers to consume interface-based row types instead of pgx.Rows.
• Updated LoadMPLobbyingSummary to use Query + Next() row handling and avoid direct pgx.ErrNoRows checks.
• Added a main package adapter (newLobbyingQueryer) so existing *pgx.Conn service wiring can still satisfy the new repository contract.
• Updated repository unit/integration tests to use the new queryer contract.

Validation

• Not run (per handoff constraints): go test ./... and go build ./....
• Not run: full backend/lobbying go test and build in this pass.
• Performed focused code review on changed files and a final diff sanity pass.

Notes

• grep -r "jackc/pgx" backend/lobbying remains non-zero in this PR because pgx remains intentionally present in existing adapter/test files outside this issue scope, to be finalized in EPAC-2220.

Linear

• https://linear.app/riddimsoftware/issue/EPAC-2222/replace-pgx-typed-queryer-interface-with-databasesql-compatible-port