- Incremental now does smart early-stopping like music: skips already-known
items and stops after 25 consecutive known (server lists recent first),
instead of a blind fixed cap. Falls back to a full pass when the library is
near-empty (<50), matching music's small-DB behavior.
- Deep scan gains music's 50% safety threshold: if removal would wipe >50% of a
>100-row library, it skips (assumes a partial server response, not a real
emptying) — prevents catastrophic deletion.
- Full Refresh already matched (re-read all, upsert, no removal).
Added DB helpers (server_ids, table_count). Tests: early-stop skips known,
small-lib fallback, 50% prune safety. 122 tests green.
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