docs(legibility): add 18 per-domain README.md files (DOC-6)

Backend (9): document, person, tag, user, geschichte, notification,
ocr, audit, dashboard.
Frontend (8): document, person, tag, user, geschichte, notification,
ocr, shared.
OCR service (1): ocr-service/README.md.

Each README covers: what the domain owns, explicit non-ownership,
public surface (verified by grep against the codebase), internal
layout, and cross-domain dependencies.

Closes #400
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/notification/README.md

@@ -0,0 +1,36 @@
+# notification (frontend)
+
+Bell-icon dropdown and real-time SSE connection for in-app notifications.
+
+## What this domain owns
+
+Components: `NotificationBell.svelte`, `NotificationDropdown.svelte`.
+Utilities: `notifications.svelte.ts` (Svelte 5 reactive store), `notifications.ts` (API helpers).
+
+## What this domain does NOT own
+
+- SSE infrastructure — the backend's `SseEmitterRegistry` manages the server-side emitter. The frontend establishes one `EventSource` connection per session. Connection management lives in `notifications.svelte.ts`.
+- Notification content rendering — notification payloads contain a `contextUrl`; the frontend navigates there on click.
+
+## Key design: SSE connection
+
+The SSE path is **backend → browser directly** (not proxied through SvelteKit SSR). The `EventSource` connects to `/api/notifications/stream`. On receive, the reactive store updates the unread count and the bell dropdown in real time.
+
+```
+Backend SseEmitterRegistry → /api/notifications/stream → EventSource in browser
+```
+
+## Key components
+
+| Component                     | Used in                       | Notes                                                     |
+| ----------------------------- | ----------------------------- | --------------------------------------------------------- |
+| `NotificationBell.svelte`     | global nav (`+layout.svelte`) | Bell icon with unread badge; opens `NotificationDropdown` |
+| `NotificationDropdown.svelte` | global nav                    | Scrollable list of recent notifications with mark-read    |
+
+## Cross-domain imports
+
+- `shared/primitives/` — icon, button primitives only
+
+## Backend counterpart
+
+`backend/src/main/java/org/raddatz/familienarchiv/notification/README.md`