docs: README discoverability rework + OG metadata + planning audit reports

README:
  - Add a "How it compares" matrix against Withings web, Apple Health,
    Oura, Garmin, and generic CSV. The audit found this section absent;
    self-hosters arrive comparing alternatives and bounce when there's
    no head-to-head.
  - Promote Apple Health import + AI Coach as standalone Key-Features
    bullets. Apple Health was the v1.4.34 banner feature with zero
    grep matches in the README until now; the audit ranked it the
    single highest-intent search keyword for ex-iOS users. AI Coach
    + Insights moved out from under "Multi-Provider AI Insights" into
    a dedicated bullet that names the Coach, Daily Briefing, Weekly
    Report, and Health Score surfaces.
  - Status block under "What it is" so visitors see release cadence
    + current focus (native iOS client) without trawling the changelog.
  - Badge row: drop "PRs welcome" + duplicate Self-Hosted, add a live
    "Latest release" badge + a GHCR multi-arch badge so the landing
    page surfaces version + container distribution at a glance.
  - Tagline tightened to lead with "self-hosted health tracker" + the
    two integration anchors (Withings + Apple Health) — both phrases
    are search-keyword fronts the GitHub search index picks up.
  - Drop the dead SESSION_SECRET env entry from Quick Start (3
    secrets, not 4) and the Environment Variables table — the env
    var was documented as required but never read by code (per the
    code-quality audit). Companion env-removal in docker-compose /
    .env.example / entrypoint ships in a parallel batch.
  - Add a one-line OpenAPI pointer above the API-Reference table so
    integrators discover the codegen source.

Layout / OG metadata:
  - openGraph + twitter cards now carry images, a higher-intent
    description, and a summary_large_image card. Uses the existing
    logo-readme.png as the OG asset; replace with a 1200x630 dashboard
    capture when one ships.

GitHub repo metadata (via gh CLI, no git diff):
  - description tightened from 290 chars to 156 chars, keyword-front.
  - homepage flipped from a personal tenant to demo.healthlog.dev so
    sidebar clicks land on a working demo instead of a login wall.
  - Topics: dropped 6 generic/PII-adjacent entries (glp-1, mounjaro,
    tracking, health, dashboard, bloodpressure), added 6 high-intent
    ones (apple-health-import, withings-alternative, glucose-tracker,
    mood-tracker, ai-insights, personal-dashboard).

Planning artifacts:
  - 7 audit reports drop into .planning/round-audit-*.md covering
    mobile-security, UX, code-quality, README/discoverability,
    perf, mobile-deep (pending), test-coverage, docs-depth. These
    are the working notes for the v1.4.34.4 audit-driven release.

src/app/layout.tsx

@@ -49,17 +49,29 @@ export const metadata: Metadata = {
   openGraph: {
     title: "HealthLog",
     description:
-      "Personal health tracking — weight, blood pressure, pulse, mood, medication compliance",
+      "Self-hosted health tracker. Weight, blood pressure, glucose, mood, medications. Withings + Apple Health sync. AI Insights you own.",
     type: "website",
     locale: "en_US",
     alternateLocale: ["de_DE", "fr_FR", "es_ES", "it_IT", "pl_PL"],
     siteName: "HealthLog",
+    // Drop-in OG asset. Replace with a 1200×630 dashboard-screenshot
+    // capture when an official one ships; the logo render keeps the
+    // unfurl from rendering a blank tile in the meantime.
+    images: [
+      {
+        url: "/logo-readme.png",
+        width: 1000,
+        height: 1000,
+        alt: "HealthLog — your health data, your server",
+      },
+    ],
   },
   twitter: {
-    card: "summary",
+    card: "summary_large_image",
     title: "HealthLog",
     description:
-      "Personal health tracking — weight, blood pressure, pulse, mood, medication compliance",
+      "Self-hosted health tracker. Weight, blood pressure, glucose, mood, medications. Withings + Apple Health sync. AI Insights you own.",
+    images: ["/logo-readme.png"],
   },
 };