docs: add navigational READMEs to key code directories

Add a README to src, src/lib, src/app, src/components, prisma, scripts,
and e2e so a technical visitor browsing the tree gets oriented: what
lives in each directory, the most important files, the local
conventions, and pointers to related dirs and docs.

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

src/README.md

@@ -0,0 +1,26 @@
+# `src/`
+
+Application source. HealthLog is a Next.js 16 App Router project (`output: "standalone"`, React 19, TypeScript strict). The path alias `@/*` resolves to this directory. Everything a request touches lives under here; the layout below follows the request flow rather than alphabetical order.
+
+## Request flow
+
+```
+client → src/proxy.ts → src/app/<route>/route.ts → src/lib/api-handler.ts → handler body → src/lib/api-response.ts → client
+```
+
+- **`proxy.ts`** — the HTTP edge (Next.js 16's renamed `middleware`). Public-path allowlist, demo-mode mutation block, server-side onboarding redirect, worker-only refusal, `x-request-id` + CSP nonce generation, every security header.
+- **`app/`** — App Router pages (RSC by default) and API routes. Every API route wraps its handler in `apiHandler`. See [`app/README.md`](app/README.md).
+- **`lib/`** — the server/runtime toolkit: `apiHandler`, auth, crypto, rollups, AI, notifications, pg-boss jobs, integrations, rate limiting. See [`lib/README.md`](lib/README.md).
+- **`components/`** — React components (shadcn/ui new-york, kebab-case files). See [`components/README.md`](components/README.md).
+
+## Other directories
+
+- **`hooks/`** — shared React hooks (`use-` prefix, kebab-case).
+- **`types/`** — shared TypeScript declarations.
+- **`generated/prisma/`** — the generated Prisma client (do not edit; regenerated by `prisma generate`). Imported as `@/generated/prisma`.
+- **`instrumentation.ts`** — Next.js startup hook; boots the pg-boss worker registry and boot-time backfill queues.
+- **`__tests__/`** — cross-cutting unit tests; most tests live in `__tests__/` folders next to the code they cover.
+
+## Conventions
+
+See [`../CLAUDE.md`](../CLAUDE.md) for the full set. The load-bearing ones: RSC by default (`"use client"` only when a hook/state/browser API needs it); every API route wraps `apiHandler` and parses its body with Zod `safeParse`; `userId` is always narrowed from session or Bearer, never accepted as a body field; TanStack Query keys come only from `src/lib/query-keys.ts`; outbound HTTP goes through `src/lib/safe-fetch.ts`.