Source Content Storage — SurrealDB-Primary, Local as a Per-User Toggle

Flip the content store: the fetched article body becomes a field on the canonical `sources` row and analyst Extracts become per-`source_usages` — so content access unifies with canonical-layer access, both through the didi.sh-authed DB. The local filesystem write demotes to an explicit per-workspace + per-user toggle: an export for each collaborator's own local RAG, not the primary store. R2 (backup) and Syncthing (live FS mirror) stay as already-scoped substrate.