[gedaiu]

Slice 3 of 5 of RC-003 (a source may decay), on top of #19. A search that hands back a document of a source with decay set records the use; nothing reads the record yet, so ranking does not change.

What is in it.

• decay::record_retrievals: one statement for all the documents of a read. For a document of a decaying source it adds one to retrieval_count, sets last_retrieved_at, and adds 2 days to the half-life up to 365 (a first retrieval starts from the source's half_life_days). Documents of other sources get no row.
• POST .../search and the search_chunks tool record what they return, spawned off the request path: a failure is logged and never fails the read. The REST body takes record: false for probes, evals and health checks. Chat's automatic prefetch does not count.
• docs/design/chat-and-mcp.md says so.

Tests. A store test pins: only the decaying source's document is written, +2 days per retrieval, the 365 cap, and an unknown id or an empty list writes nothing. A unit test pins that record defaults to true.

Checked locally on the rebased branch: cargo fmt --check, cargo clippy --workspace --all-targets -D warnings, cargo test --workspace (1684 passed, 0 failed).

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