feat(insights): per-score assessment text + shared metric signal block

Add an additive, non-breaking assessment field to GET /api/insights/derived:
{ text, source, updatedAt } | null, keyed to the requested metric id for the
five per-score ids (READINESS, SLEEP_SCORE, RECOVERY_SCORE, STRAIN_SCORE,
STRESS_SCORE). The field is null when the metric is not one of those or when
its status is not "ok". A deterministic text always fills it — composed from
the score's own contributors, so provider-less accounts and the demo are never
empty — and warmer provider prose overrides it once cached, warmed out of band
so the GET never blocks on a model round-trip. de/en, following the v1.12.12
deterministic-narrative model. The batch grid read returns assessment: null;
the detail sheets use the single-metric route.

Introduce a shared buildMetricSignal() helper: a token-lean, signal-rich
descriptor (current, own baseline, signed delta, spread, normal-swing verdict,
normal-range placement, contributors) computed server-side so the model states
the comparison instead of re-deriving it from raw buckets. Thread it into the
generic per-metric assessment snapshot and lead the base prompt from it. Add a
PROMPT_VERSION constant to the base system prompt so prompt changes are
traceable.

Regenerate docs/api/openapi.yaml.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/openapi/routes.ts

@@ -1445,6 +1445,26 @@ const derivedProvenance = z
   })
   .meta({ id: "DerivedProvenance" });
 
+// v1.13.2 — per-derived-SCORE assessment text. Additive, non-breaking field
+// on the derived response; the iOS field-name contract is LOCKED.
+const derivedAssessment = z
+  .object({
+    text: z
+      .string()
+      .describe(
+        "Short, non-empty explanation of why the score sits where it does, referencing the score's contributors.",
+      ),
+    source: z
+      .string()
+      .describe(
+        "'deterministic' for the always-on template text, or 'ai' when warmer provider prose has been cached.",
+      ),
+    updatedAt: z.iso
+      .datetime({ offset: true })
+      .describe("When the text was produced / last warmed."),
+  })
+  .meta({ id: "DerivedAssessment" });
+
 const derivedMetricResponse = z
   .object({
     metric: z
@@ -1470,6 +1490,11 @@ const derivedMetricResponse = z
       .string()
       .nullable()
       .describe("Why the value could not be produced; null when status is 'ok'."),
+    assessment: derivedAssessment
+      .nullable()
+      .describe(
+        "v1.13.2 — short 'why is this score what it is' explanation, keyed to the SAME requested id (only for the per-score ids READINESS, SLEEP_SCORE, RECOVERY_SCORE, STRAIN_SCORE, STRESS_SCORE). Null for any other metric and whenever status !== 'ok'. Always non-empty when present: a deterministic text fills it (so provider-less accounts + the demo always get one) and warmer AI prose overrides it once cached.",
+      ),
   })
   .meta({
     id: "DerivedMetricResponse",