fix(control-plane): coalesce dates for timelines

[mdbenito]

The entity timeline in the control plane plotted each memory only by occurred_start, a content date the LLM sets on a small fraction of memories, so most entities showed "No Timeline Data" despite every memory carrying mention and ingest timestamps.

This PR makes it so that timelines plot each row by an effective date computed in the UI as occurred_start → mentioned_at → occurred_end → created_at → event_date, replicating the backend's COALESCE order for a unit's effective time and extending it with system fields that are always populated. Only rows with no usable date at all are excluded. Filtering, sorting, grouping, and labels all use this date.

Additionally:

• the date partition and granularity grouping are extracted into pure helpers (with unit tests)\
• the empty-state message was reworded across locales
• the graph endpoint's non-ISO YYYY-MM-DD HH:MM date is parsed explicitly as UTC since it falls outside the ECMAScript date grammar.

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Note: The backend actually shows two different date policies (this PR follows the 2nd one):

1. "Effective time": COALESCE(occurred_start, mentioned_at, occurred_end), used by temporal retrieval (retrieval.py::_coalesce_date) and recency reranking (reranking.py:156, but inlined instead of calling the helper).
2. Display/any-time: the extended chain including created_at/event_date, used by get_memories_timeseries and now by the timeline view.

This PR adds more behaviour by handling the graph endpoint's non-ISO date format, which no backend helper currently does, but it does mean the display chain now exists in three places.

Two follow-ups worth considering:

1. Move _coalesce_date to a shared module and have reranking.py use it, eliminating the inline duplicate.
2. Compute effective_date server-side on /list and /graph endpoints