#523 - Reduce any usage in db2 model/query layer

Parent: #523

## Objective
Improve data-layer correctness by reducing broad `any` usage and introducing stronger query/model typings.

## In scope
- Start with high-traffic entities (e.g., plan/user) in db2 model/query paths
- Add typed DTO/query result contracts for selected models
- Refactor shared model base utilities where needed to support stronger typing

## Acceptance criteria
- Measurable reduction of `any` in touched db2 files
- Typed create/update/find paths for selected entities
- No query behavior regressions in existing tests
- Additional tests for edge cases introduced by typing changes

## Notes
Deliver incrementally to avoid high-risk all-at-once migration.