[AZ-486] F7 endpoint builders + STC-ARCH-02 (cycle 1 close)

Single source of truth for every /api/<service>/... URL the UI talks to:
src/api/endpoints.ts (25 typed builders) re-exported via the F4 barrel.
Migrates 13 production callsites in admin / annotations / flights /
settings / dataset / auth / api-client / FlightContext / DetectionClasses
to endpoints.* . Adds the STC-ARCH-02 static gate (--mode=api-literals
in scripts/check-arch-imports.mjs, wired into scripts/run-tests.sh)
that fails any new hardcoded /api/<service>/ literal in src/ outside
endpoints.ts and *.test.tsx? files.

Tests: +36 contract assertions in src/api/endpoints.test.ts (every
builder, character-identical), +6 STC-ARCH-02 architecture cases in
tests/architecture_imports.test.ts (single / double / template literal
fail paths, *.test.* exemption, line-comment skip, migrated codebase
pass). Fast profile 167 -> 209 PASS / 13 SKIP / 0 FAIL, +42 new,
0 regressions. Static profile 31 / 31 PASS.

Closes architecture baseline finding F7. Cycle 1 of Phase B closed.

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

src/features/annotations/VideoPlayer.tsx

@@ -1,5 +1,6 @@
 import { useRef, useState, useCallback, useEffect, forwardRef, useImperativeHandle } from 'react'
 import { FaPlay, FaPause, FaStop, FaStepBackward, FaStepForward, FaVolumeMute, FaVolumeUp } from 'react-icons/fa'
+import { endpoints } from '../../api'
 import type { Media } from '../../types'
 
 interface Props {
@@ -38,7 +39,7 @@ const VideoPlayer = forwardRef<VideoPlayerHandle, Props>(function VideoPlayer({
 
   const videoUrl = media.path.startsWith('blob:')
     ? media.path
-    : `/api/annotations/media/${media.id}/file`
+    : endpoints.annotations.mediaFile(media.id)
 
   const stepFrames = useCallback((count: number) => {
     const video = videoRef.current