docs(08-04/05/06): add plan summaries — wave 2 backend decomposition complete

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

.planning/phases/08-stack-upgrade-backend-decomposition/08-04-SUMMARY.md

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+# Plan 08-04 Summary — Admin API Decomposition
+
+**Status:** Complete  
+**Requirements:** CODE-01, CODE-08  
+**Date:** 2026-06-12
+
+## What Was Done
+
+- Added `validate_provider_id()` to `backend/services/ai_config.py` (D-11 migration)
+- Created `backend/api/admin/` package: `shared.py`, `users.py`, `quotas.py`, `ai.py`, `__init__.py`
+- `__init__.py` aggregates with `prefix="/api/admin"` — 13 routes total
+- All sub-routers declare no prefix (D-04)
+- `_user_to_dict` shared helper in `shared.py` (T-02-27 / SEC-07 field whitelist)
+- Both `SystemAiConfigUpdate` and `TestConnectionRequest` call `validate_provider_id()` (CODE-08)
+- Deleted `backend/api/admin.py` monolith after 54-test URL regression passed
+
+## Test Results
+
+- `tests/test_admin_api.py`: 27 passed
+- `tests/test_cloud.py` + `tests/test_admin_ai_config.py`: 27 passed
+- Full suite: 405 passed (1 pre-existing docx env skip)
+
+## Admin Paths (unchanged)
+
+`/api/admin/users`, `/api/admin/users/{id}`, `/api/admin/users/{id}/status`, `/api/admin/users/{id}/quota`, `/api/admin/users/{id}/ai-config`, `/api/admin/users/{id}/password-reset`, `/api/admin/topics`, `/api/admin/ai-config`, `/api/admin/ai-config/models`, `/api/admin/ai-config/test-connection`