fix(dashboard): make Top Senders use highest-count facet values

[CafeLungo]

Problem

The "Top 10 Senders" widget on the dashboard showed wildly inaccurate counts. On a
~200k-email archive the top bar maxed out around 25 — effectively a random/low set
of senders rather than the real leaders.

Root cause

SearchService.getTopSenders() reads Meilisearch's from facet distribution
(index.search('', { facets: ['from'] })). But configureEmailIndex() never set any
faceting config, so Meilisearch applied its defaults:

• maxValuesPerFacet: 100 — only 100 of the many thousands of distinct senders are returned
• sortFacetValuesBy: 'alpha' — those 100 are the alphabetically-first addresses, not the most frequent

getTopSenders() then sorted that arbitrary alphabetical slice by count and took the
top 10 — so it surfaced the top of an alphabetical subset, never the true high-volume
senders. The same defect affected getFacetValues() (facet typeahead suggestions).

Fix

Configure faceting on the emails index to sort facet values by count and raise the cap:

faceting: {
    maxValuesPerFacet: 1000,
    sortFacetValuesBy: { '*': 'count' },
},

Meilisearch now returns the highest-frequency facet values, so the widget sees the
real leaders. Settings-only change, applied at startup via configureEmailIndex() —
no reindex required.

Verification

Rebuilt and ran the stack against a 201,598-email index. Meilisearch's facet
distribution now matches the Postgres ground truth almost exactly:

Sender	Widget (Meilisearch)	Postgres (truth)
store-news@amazon.com	8,338	8,340
notification@***.com	7,178	7,179
***@gmail.com	5,231	5,252
noreply@***.com	4,979	4,979
enews@***.com	4,245	4,245

(Sub-handful differences are docs still in-flight through indexing, or negligible at least.)