chore(release): v1.12.12 — deterministic fallback so the period retrospective is never empty

When no AI provider is configured (or before the first AI summary is written),
the period-in-review card stayed empty. Compose a concise, factual, strictly
non-causal summary from the same structured digest the AI prompt is built from
— ranked metric changes vs the prior period, vitals that moved outside their
typical range, and an honest count of statistical associations — and persist it
with a `deterministic` provider marker so a later AI warm replaces it in place.
The read route now warms regardless of provider; the read-never-blocks posture
is unchanged. Fills the no-key demo's retrospective too.

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

docs/api/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.12.11
+  version: 1.12.12
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.