[Phase 0] Add dbt sources and staging models for OpenFootball

[dcaribou]

Type: feature
Suggested labels: phase-0, dbt, new-source

Create source definitions and staging models (stg_openfootball_*) that normalize raw OpenFootball records to canonical intermediate columns.

Acceptance criteria:

• dbt source YAML for openfootball exists.
• Staging models for competitions/clubs/games build successfully.
• Staging tests cover PK uniqueness, required fields, and date parse validity.

Dependencies: #3