feat(insights): add the vitals dashboard + its derived metric tier

Build the Apple-Health-Highlights grid of personal-vital tiles on the
Insights overview, backed by four new pure derived-metric compute
functions over the established seam:

- cardio-fitness band ("Fitness Age" frame) — VO2max placed against the
  FRIEND-registry age/sex norm (Kaminsky 2015), surfaced + trended, never
  re-derived.
- vascular-age delta — Withings vascular-age vs chronological age, a
  relative wellness indicator (Vlachopoulos 2010 PWV framing), no medical
  claim.
- HRV (SDNN) balance — reuses the typical-range engine for the band and
  places the recent average as balanced / above / below; SDNN never
  relabelled RMSSD; cadence-gated.
- BMI from weight + height — exact WHO classification, the fallback when
  no device BMI exists.

Each metric carries coverage + confidence + provenance and cites its
standard through the provenance explainer. The dashboard renders one
SparklineDeltaTile per available vital: an absent vital does not render,
a still-building one shows the honest coverage meter rather than a
headline from too little history. Tiles read the generic
/api/insights/derived route through a TanStack Query hook keyed off the
centralised factory.

Regenerates the OpenAPI contract for the two new metric ids and adds the
vitals copy across all six locales.

docs/api/openapi.yaml

@@ -2330,13 +2330,15 @@ paths:
               - SLEEP_SCORE
               - READINESS
               - COINCIDENT_DEVIATION
-            description: "Derived-metric id to compute (e.g. VITALS_BASELINE, FITNESS_AGE, READINESS). Closed enum: an unknown id
-              422s. Wave 1 implements VITALS_BASELINE end-to-end; not-yet-implemented metrics return an `insufficient`
-              value with reason `not_implemented`."
+              - HRV_BALANCE
+              - BMI
+            description: "Derived-metric id to compute (e.g. VITALS_BASELINE, FITNESS_AGE, VASCULAR_AGE_DELTA, HRV_BALANCE, BMI,
+              READINESS). Closed enum: an unknown id 422s. Metrics whose compute has not yet landed return an
+              `insufficient` value with reason `not_implemented`."
           required: true
-          description: "Derived-metric id to compute (e.g. VITALS_BASELINE, FITNESS_AGE, READINESS). Closed enum: an unknown id
-            422s. Wave 1 implements VITALS_BASELINE end-to-end; not-yet-implemented metrics return an `insufficient`
-            value with reason `not_implemented`."
+          description: "Derived-metric id to compute (e.g. VITALS_BASELINE, FITNESS_AGE, VASCULAR_AGE_DELTA, HRV_BALANCE, BMI,
+            READINESS). Closed enum: an unknown id 422s. Metrics whose compute has not yet landed return an
+            `insufficient` value with reason `not_implemented`."
         - in: query
           name: type
           schema:
@@ -7332,6 +7334,8 @@ components:
             - SLEEP_SCORE
             - READINESS
             - COINCIDENT_DEVIATION
+            - HRV_BALANCE
+            - BMI
           description: Echoes the requested derived-metric id (tags the union).
         status:
           type: string