Adding a data provider

A provider is a Go package that implements provider.Provider (internal/provider/provider.go):

type Provider interface {
    Name() string            // stored with every raw payload, e.g. "sportmonks"
    ParserVersion() string   // bump when parsing the same payload can give different output

    FetchSeason(ctx, seasonRef) (Raw, error)    // I/O
    ParseSeason(Raw) (Season, error)            // pure
    FetchSchedule(ctx, seasonRef) (Raw, error)
    ParseSchedule(Raw) (Schedule, error)        // matchday number -> fixture refs
    FetchFixture(ctx, fixtureRef) (Raw, error)
    ParseFixture(Raw) (Fixture, error)          // the important one
}

Steps:

1. Create internal/provider/<name>/. Use httpx.Client for requests: it handles timeouts, retries with backoff, Retry-After and client-side rate limiting. If the provider reports back-off somewhere other than the Retry-After header, set RetryAfter.
2. Keep Fetch and Parse separate. Fetch returns the response body verbatim. Parse must be pure, because stored payloads are re-parsed later without the network.
3. ParseFixture produces domain.MatchFacts using the provider’s own IDs: lineups with roles and starters, optional ratings, and the events that matter (goals, penalties scored, missed or saved, own goals, substitutions, cards). Order events with Seq. Skip rescinded events and penalty shoot-outs. Set Final only once the match is over.
4. Add recorded responses under testdata/ with a golden test, and check that timeline.Derive accepts the parsed facts. The Sportmonks package is a complete example.
5. Wire it in cmd/openfanta (today the Sportmonks client is created in newService).

Ingest owns caching, ID mapping (provider IDs never leak into the API) and versioning, so a provider needs none of that.