refactor(settings): station.settings SQL moves into StationSettingsRepository (gh-#406 slice 3)

[genwave-radio]

gh-#406 L2 burn-down, slice 3 of 5 — stacked on slice 2 (merge in order; merge commits, not squash). Slices 4 and 5 build on this seam.

⚖️ New StationSettingsRepository (GenWave.MediaLibrary.Station) owns the station.settings row I/O with byte-identical SQL. Deliberate deviation from the Lazy<NpgsqlDataSource> wiring pattern: the ctor takes the plain connection string (documented in its remarks) — slices 4/5 need it constructible before and outside the DI container for the pre-DI IConfigurationProvider boot path. StationSettingsStore keeps everything that is genuinely its own — the write-side allowlist guard, reload signal, change event, empty-connection-string degrade — and delegates the rows; no DI wiring change (StationSettingsHostingExtensions untouched).

📝 One necessary posture translation: the store's read-side degrade catch was NpgsqlException; it becomes DbException (verified: NpgsqlException : DbException in the pinned Npgsql 10.0.3 — same failures caught, OperationCanceledException still propagates), since Host may no longer name Npgsql types. Documented in the method remarks.

📉 Baseline row deleted (3 debt rows → 2: only the Provider and SeedMarker rows remain, for slices 5 and 4).

🧪 New db-backed Story042_StationSettingsRepository specs: insert, upsert-in-place, updated_at advance, unfiltered read-all, case-insensitive lookup, round trip, PK-null sad path.

✅ At stack tip: build 0 warnings; Architecture 104/104; MediaLibrary 763/763; Host 1723/1723 (settings overlay coverage unchanged and green); Core 144/144.