feat(stats): add time_field toggle to memories-timeseries chart

[aliu-ronin]

Summary

The Memories ingested chart on the bank dashboard always buckets by memory_units.created_at (ingest time). For a bank built up in real time, ingest time ≈ event time and that's the right default. But when a corpus is backfilled in a single session — for example migrating from another memory system — every record's created_at collapses to the import moment, so the chart shows "all knowledge is new" and the underlying timeline disappears.

Adds a time_field query parameter (and a three-way UI toggle) that lets the viewer pick which timestamp column drives the bucket assignment:

Value	Meaning
created_at (default)	Ingest time — matches old behavior exactly
mentioned_at	Event time: when the fact was mentioned
occurred_start	Event time: when the underlying event started

For the event-time columns we COALESCE(<col>, created_at) per row so records lacking an event timestamp still show up in the chart instead of silently disappearing. The field is whitelisted (never interpolated from untrusted input), unknown values fall back to created_at, and the chosen column is echoed back in the response for UI affordance.

Changes

Backend (hindsight-api-slim)

• memory_engine.get_memories_timeseries gains a time_field keyword arg, validates it against an allowlist, and composes bucket_expr with COALESCE for the event-time columns.
• MemoriesTimeseriesResponse adds a time_field field.
• API endpoint exposes time_field as a query param with a clear description.

Control Plane (hindsight-control-plane)

• getMemoriesTimeseries SDK helper accepts a timeField argument, defaulting to created_at.
• /api/stats/[agentId]/memories-timeseries proxy forwards the param.
• bank-stats-view.tsx renders an Ingested / Mentioned / Occurred segmented control below the period selector; the card title updates so the chart reads unambiguously (e.g. "Memories by mentioned time").

Screenshots

On a bank migrated from another memory system, toggling to Mentioned spreads the chart across the actual event timeline instead of a single ingest-day spike:

• created_at → two-day spike (04-23, 04-24)
• mentioned_at → spread across 03-26 through 03-29 (real history)

Test plan

• curl .../memories-timeseries?period=30d&time_field=mentioned_at returns non-zero buckets spanning the corpus's actual timeline
• time_field=nonsense falls back to created_at (parity with the invalid-period behavior)
• Response includes time_field so the UI can display the active dimension
• Control plane chart updates immediately when the toggle changes, with no stale frames

Notes

• Defaults stay created_at everywhere; existing integrations see no behavioral change.
• Builds on fix(stats): timeseries buckets should return tz-aware ISO #1245 (tz-aware bucket ISO). Without that fix the chart is off-by-timezone in non-UTC browsers regardless of which time_field is selected.

[nicoloboschi]

LGTM

[nicoloboschi]

@aliu-ronin can you resolve conflicts?