[AZ-485] Add Public API barrels + STC-ARCH-01 (F4 close)

Closes architecture baseline finding F4. Every component now exposes
its Public API through `src/<component>/index.ts`; cross-component
imports go through the barrel. `scripts/check-arch-imports.mjs` plus
`STC-ARCH-01` in the static profile enforce the rule; tests in
`tests/architecture_imports.test.ts` cover AC-4/AC-5 + 2 exemption
cases. One F3-pending exemption (`classColors`) is documented in 5
places (barrel, consumer, script, doc, test) to avoid a circular
import.

Phase B cycle 1 batch 1 of 2 (epic AZ-447). Batch 2 is AZ-486
(endpoint builders) — blocked on this commit landing.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/features/annotations/AnnotationsSidebar.tsx

@@ -1,8 +1,7 @@
 import { useEffect, useState } from 'react'
 import { useTranslation } from 'react-i18next'
 import { FaDownload } from 'react-icons/fa'
-import { api } from '../../api/client'
-import { createSSE } from '../../api/sse'
+import { api, createSSE } from '../../api'
 import { getClassColor } from './classColors'
 import type { Media, AnnotationListItem, PaginatedResponse } from '../../types'