feat(ingestion): add persisted source model foundation

[alucero270]

Summary

• add a first-class sources table for configured ingestion sources with filesystem-focused fields for name, type, root path, status, and last-indexed metadata
• ensure the configured filesystem source is persisted through the existing ingestion path without introducing client-controlled source scope
• document the current source-model foundation and the optional STRATA_SOURCE_NAME configuration surface

Validation

• dotnet build src/Codex.Api/Codex.Api.csproj '-p:OutDir=C:/Users/Alex Lucero/source/repos/strata/artifacts/validation/api/out/'
• dotnet build src/Codex.Indexer/Codex.Indexer.csproj '-p:OutDir=C:/Users/Alex Lucero/source/repos/strata/artifacts/validation/indexer/out/'
• git diff --check

Notes

• this PR introduces the persisted source-model foundation only; it does not yet add source-scoped document links, source-scoped job ownership, or source management APIs
• source records remain server-controlled and derive from deployment configuration, not from client requests

Closes #26