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/eslint.config.js

@@ -3,6 +3,7 @@ import { fileURLToPath } from 'node:url';
 import { includeIgnoreFile } from '@eslint/compat';
 import js from '@eslint/js';
 import svelte from 'eslint-plugin-svelte';
+import boundaries from 'eslint-plugin-boundaries';
 import { defineConfig } from 'eslint/config';
 import globals from 'globals';
 import ts from 'typescript-eslint';
@@ -61,5 +62,84 @@ export default defineConfig(
 				}
 			]
 		}
+	},
+	{
+		plugins: { boundaries },
+		settings: {
+			'import/resolver': { typescript: { project: './tsconfig.json' } },
+			'boundaries/elements': [
+				{ type: 'document', pattern: 'src/lib/document/**' },
+				{ type: 'person', pattern: 'src/lib/person/**' },
+				{ type: 'tag', pattern: 'src/lib/tag/**' },
+				{ type: 'user', pattern: 'src/lib/user/**' },
+				{ type: 'geschichte', pattern: 'src/lib/geschichte/**' },
+				{ type: 'notification', pattern: 'src/lib/notification/**' },
+				{ type: 'ocr', pattern: 'src/lib/ocr/**' },
+				{ type: 'activity', pattern: 'src/lib/activity/**' },
+				{ type: 'conversation', pattern: 'src/lib/conversation/**' },
+				{ type: 'shared', pattern: 'src/lib/shared/**' },
+				{ type: 'routes', pattern: 'src/routes/**' }
+			]
+		},
+		rules: {
+			'boundaries/dependencies': [
+				'error',
+				{
+					default: 'disallow',
+					message:
+						"Cross-domain import blocked. Move shared code to $lib/shared/, or expose it via the domain's index.ts.",
+					rules: [
+						// Document composes person components (D-FE-1) and tag components (D-FE-2),
+						// and hosts the OCR trigger in the transcription editor.
+						{
+							from: { type: 'document' },
+							allow: {
+								to: { type: ['shared', 'conversation', 'activity', 'person', 'tag', 'ocr'] }
+							}
+						},
+						// Geschichte editor selects persons and documents by design.
+						{
+							from: { type: 'geschichte' },
+							allow: { to: { type: ['shared', 'person', 'document'] } }
+						},
+						// OCR trigger embeds the document script-type selector.
+						{
+							from: { type: 'ocr' },
+							allow: { to: { type: ['shared', 'document'] } }
+						},
+						// Activity feed (Chronik) reads notification items for its inbox panel.
+						{
+							from: { type: 'activity' },
+							allow: { to: { type: ['shared', 'notification'] } }
+						},
+						{ from: { type: 'person' }, allow: { to: { type: ['shared'] } } },
+						{ from: { type: 'tag' }, allow: { to: { type: ['shared'] } } },
+						{ from: { type: 'user' }, allow: { to: { type: ['shared'] } } },
+						{ from: { type: 'notification' }, allow: { to: { type: ['shared'] } } },
+						{ from: { type: 'conversation' }, allow: { to: { type: ['shared'] } } },
+						{ from: { type: 'shared' }, allow: { to: { type: ['shared'] } } },
+						{
+							from: { type: 'routes' },
+							allow: {
+								to: {
+									type: [
+										'document',
+										'person',
+										'tag',
+										'user',
+										'geschichte',
+										'notification',
+										'ocr',
+										'activity',
+										'conversation',
+										'shared'
+									]
+								}
+							}
+						}
+					]
+				}
+			]
+		}
 	}
 );

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';