feat(mcp): add a local stdio MCP server for read-only health data

Introduce a read-only Model Context Protocol surface built on the official
@modelcontextprotocol/sdk so a user can query their own HealthLog data from an
MCP-capable assistant over local stdio with a pasted, scoped Bearer token — no
extra infrastructure.

The tool and resource layer in src/lib/mcp is transport-agnostic: five read
tools — list_metrics, get_metric_series, get_medication_compliance, get_labs,
get_correlations — each a thin wrapper over the existing server-authoritative
retrieval paths, returning structured values, units, reference bands, and
provenance with an explicit { present: false } for absence and no prose
verdicts. Two resources expose the user's profile (data-minimised) and
medication list. A stdio entrypoint (src/cli/mcp-stdio.ts, pnpm mcp:stdio)
resolves the token through the shared Bearer path and binds the session to
<user>:<token>; with no token it refuses to start.

Read-only and admin-unreachable by construction: only read tools are
registered, and requireAdmin stays cookie-only so the Bearer/MCP wire can never
elevate. Covered by unit tests for every tool and resource, the auth/scope
model, and the cookie-only admin boundary. The server ships in the existing
container; docker compose up is unchanged.

package.json

@@ -47,7 +47,8 @@
     "e2e:ui": "playwright test --ui",
     "openapi:generate": "npx tsx scripts/generate-openapi.ts",
     "openapi:check": "npx tsx scripts/check-openapi.ts",
-    "check-env": "npx tsx scripts/check-env.ts"
+    "check-env": "npx tsx scripts/check-env.ts",
+    "mcp:stdio": "npx tsx src/cli/mcp-stdio.ts"
   },
   "dependencies": {
     "@aws-sdk/client-s3": "^3.1048.0",
@@ -55,6 +56,7 @@
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
     "@hookform/resolvers": "^5.4.0",
+    "@modelcontextprotocol/sdk": "^1.29.0",
     "@node-rs/argon2": "^2.0.2",
     "@parse/node-apn": "^8.1.0",
     "@prisma/adapter-pg": "^7.8.0",