feat(eslint): add boundaries/dependencies rule preventing cross-domain imports

Adds eslint-plugin-boundaries with one element type per Tier-1 domain and an
explicit allow-list encoding the architectural dependency graph:
- document may import from: shared, person, tag, ocr, activity, conversation
- geschichte may import from: shared, person, document
- ocr may import from: shared, document
- activity may import from: shared, notification
- all others (person, tag, user, notification, conversation): shared only
- routes may import from any domain

Default is 'disallow', so any unlisted cross-domain import is an error.
Two eslint-disable-next-line comments remain in shared/discussion where
person-domain helpers (getInitials, formatLifeDateRange) are needed to render
participant metadata; moving them to shared would lose the person-type context.

Closes #410
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/CommentMessage.svelte

@@ -2,6 +2,7 @@
 import { m } from '$lib/paraglide/messages.js';
 import type { FlatMessage } from '$lib/shared/types';
 import { extractQuote } from '$lib/shared/discussion/comment';
+// eslint-disable-next-line boundaries/dependencies -- discussion UI needs person initials for avatars; move to shared if getInitials becomes generic
 import { getInitials } from '$lib/person/personFormat';
 import { relativeTime } from '$lib/shared/utils/time';
 import { renderBody } from '$lib/shared/discussion/mention';

frontend/src/lib/shared/discussion/MentionDropdown.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 import type { components } from '$lib/generated/api';
+// eslint-disable-next-line boundaries/dependencies -- mention dropdown needs person date formatting; extract to shared if it becomes reusable
 import { formatLifeDateRange } from '$lib/person/personLifeDates';
 import { m } from '$lib/paraglide/messages.js';