feat(topics): compute topic overviews and topic pages on the server

GET /api/topics answers one entry per topic the record holds data for:
a headline value, a rule-based sentence and a 30-day sparkline.
GET /api/topics/{topic} answers a topic's signals with their usual
ranges, side-by-side series for a chosen window, the relationships
between its signals, 90-day trends, what it holds nothing for yet, and
its blocks (training load, lab markers with their trend, screener
trends, the cycle day).

Every figure comes from reads the metric pages already make: the local
daily values, the reconstructed nights, the mood score, computeLabTrend,
the training load and the screener trends. No model is involved. A
relationship says nothing below 14 paired days and is only called a
pattern at p < 0.05; a lab trend needs three readings spanning a year.

The preview is the viewer's own opt-in: with it off both routes answer
404 topics.preview_off. Topics, signals and blocks follow the record's
module switches and the sections the viewer's grant opens; the routes
read across sections, so a scoped grant is refused like the timeline.
/api/auth/me publishes the caller's opt-ins as previewFeatures.

src/lib/settings/preview-features.ts

@@ -6,10 +6,11 @@
  * adds it beside what is there and never replaces an existing page.
  *
  * Flags:
- * - `topics`: topic overview pages above the single-metric Insights pages.
- *   Stored, served by `/api/settings/preview-features` and carried by the
- *   backup. The Settings > System switch is not listed until the topic pages
- *   ship, because nothing reads the flag yet.
+ * - `topics`: the Topics area (`/topics`, `/api/topics`), overview pages
+ *   that bring related signals together above the single-metric Insights
+ *   pages, which stay exactly as they are. Switched in Settings > System >
+ *   Preview features, published on `/api/auth/me` as `previewFeatures`, read
+ *   by the navigation and refused by the topic routes while off.
  *
  * Unknown keys in a stored blob are dropped on read, so a flag retired later
  * disappears cleanly and a malformed blob reads as everything off.