[gedaiu]

Slice 2 of 5 of RC-003 (a source may decay), on top of #18. A source can now be told to decay over the API; nothing reads the setting yet, so no behaviour changes.

What is in it.

• PATCH /api/v1/kbs/{kb_id}/sources/{source_id} takes decay: an object turns it on (a missing field takes Lore's value, so {} is Lore's behaviour), null turns it off, leaving the field out changes nothing. The stored object always has all four fields, so a later change of the defaults does not move a source that was set.
• DecaySettings::validate: max_items and stale_after_days at least 1, half_life_days 1 to 365. A refused value changes nothing.
• Source and SourceView carry decay; the audit row says decay_changed.
• docs/design/sources.md describes the option.

Tests. A store test pins keep, set, clear, refuse and the list; a unit test pins that an update body tells a missing decay from a null one; one for the range check.

Not here. The web form: until it exists the option is set over the API. Create does not take decay either; set it after creating.

Checked locally on the rebased branch: cargo fmt --check, cargo clippy --workspace --all-targets -D warnings, cargo test --workspace (1682 passed, 0 failed).

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