Fix Phase 3 UAT blockers: MinIO presigned URL hostname, CORS, admin flush→commit, auth refresh race

Bugs fixed:
- minio_backend.py: generate_presigned_put_url and presigned_get_url used internal
  _client (minio:9000) instead of _public_client (localhost:9000). Browser received
  ERR_NAME_NOT_RESOLVED. Fixed by using _public_client with region='us-east-1' to
  skip region-discovery HTTP request from inside the container.

- docker-compose.yml: MINIO_API_CORS_ALLOW_ORIGIN was set from CORS_ORIGINS which
  uses pydantic JSON list format '["http://localhost:5173"]'. MinIO expected a plain
  string and never matched the origin. Fixed to use FRONTEND_URL instead.

- admin.py: All write handlers (create_user, update_user_status, update_user_quota,
  update_ai_config) used session.flush() without session.commit(). Changes appeared
  to succeed (response reflected in-memory state) but rolled back on session close.
  Fixed by replacing flush() with commit() in all four write handlers.

- auth.js: Concurrent refresh() calls from QuotaBar and App.vue on page reload caused
  a token rotation race — first call rotated the cookie, second arrived with stale
  cookie and cleared accessToken. Fixed by deduplicating with a shared in-flight
  promise (_refreshInFlight).

Phase 3 UAT: 9/10 pass. UAT-3 (QuotaBar visual) pending browser confirmation.

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

backend/api/admin.py

@@ -205,7 +205,7 @@ async def create_user(
         used_bytes=0,
     )
     session.add(quota)
-    await session.flush()
+    await session.commit()
 
     return {
         "id": str(new_user.id),
@@ -254,7 +254,7 @@ async def update_user_status(
         await revoke_all_refresh_tokens(session, user.id)
 
     session.add(user)
-    await session.flush()
+    await session.commit()
 
     return {
         "id": str(user.id),
@@ -346,7 +346,7 @@ async def update_user_quota(
 
     quota.limit_bytes = body.limit_bytes
     session.add(quota)
-    await session.flush()
+    await session.commit()
 
     response: dict = {
         "user_id": str(quota.user_id),
@@ -378,7 +378,7 @@ async def update_ai_config(
     user.ai_provider = body.ai_provider
     user.ai_model = body.ai_model
     session.add(user)
-    await session.flush()
+    await session.commit()
 
     return {
         "id": str(user.id),