[gedaiu]

Slice 4 of 5 of RC-003 (a source may decay), on top of #20. A source that sets decay is now kept to its max_items.

What is in it.

• decay::sources_over_cap and decay::plan_eviction: for a source over max_items, take the 2 x excess oldest live documents that are at least min_age_days old, score them with Lore's score (document name as key, text length, retrievals, half-life from document_salience or the source, stale derived from the last retrieval), and pick the excess lowest. Equal scores keep the older first.
• decay_sweep.rs: deletes each chosen document the way a person deletes one (so it can be restored), removes it from the full-text index, and writes one decay.evict audit row with no actor.
• A daily ticker (copied from the governance one) queues one decay_sweep job per source over its cap with enqueue_unless_queued, and dispatch runs it. The job has the default priority, so it waits behind extraction.
• docs/design/sources.md describes it.
• Memory now owns its key and carries a text length, and is_stale is the one staleness rule.

Tests. A unit test for the selection and one for staleness; a store test builds a source with five documents (one too young, one recently read, one with a topic key) and pins which two go; a server test runs the sweep against a real database and search index and pins the deletion, the index removal, the audit row and that a second sweep does nothing.

Size. 527 lines, about 230 of them tests. Fine for the fork; to be split before it goes upstream.

Checked locally on the rebased branch: cargo fmt --check, cargo clippy --workspace --all-targets -D warnings, cargo test --workspace (1688 passed, 0 failed).

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