# 026 — Settings: maximum logs kept in memory

## Summary

ottyl currently keeps every ingested log forever in the in-memory `BTreeMap`. The
wishlist wants a settings-configurable cap on how many logs are retained, with no
cap (unlimited) as the default.

## Scope

- New `Settings` field (e.g. `max_logs: Option<usize>`), `None`/unset = unlimited,
  matching the existing "by default it should be unlimited" requirement.
- Enforce the cap in the engine's ingest path: once the `IngestedLog` range
  exceeds the configured max, evict the oldest entries (lowest `sequence`) to
  bring it back under the cap.
- Decide and document what happens to data that references evicted logs:
  - Params/metrics already derived into `Bucket` values from an evicted log are
    **not** retroactively removed (buckets are aggregate data, not raw logs) —
    state this explicitly so it's not treated as a bug later.
  - `sequence` numbering does not reset/renumber on eviction.
- Expose the setting in the Settings API/NavMenu settings panel (a numeric input,
  clearable back to "unlimited").

## Acceptance criteria

- With no cap configured, behavior is unchanged (unbounded retention, as today).
- Setting a cap and ingesting past it keeps memory usage bounded — oldest logs
  are evicted, newest `max_logs` remain queryable via `GET /api/logs`.
- Changing the cap at runtime (via settings) takes effect on the next ingest
  without requiring a restart.

## Depends on

None — extends the existing `Settings` model.