feedsignal-db's Db impl had grown into one 250-line block mixing feed
subscriptions, article storage/scoring, reading-event logging, and
topic-affinity persistence in a single file. Split each domain's
methods into its own module (mirroring the earlier web crate SRP
split), keeping lib.rs to just the Db struct and connection setup.
Pure move — no behavior or public API changes.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QmwX8eafbMstJvt8XPSqft
