docs: add identifier field documentation and CHANGELOG for 15.0.7

- docs/data-schemas/treatments-schema.md: Document REQ-SYNC-072 identifier normalization
  - Describe how Loop/AAPS/xDrip+ sync fields are unified into identifier
  - Add example showing UUID _id → identifier promotion
  - Update sync/reconciliation fields table
- CHANGELOG.md: Create changelog for 15.0.7 release
  - Document UUID/identifier handling (REQ-SYNC-072)
  - Document test infrastructure improvements (GAP-SYNC-046)
  - Document Node.js/MongoDB requirement changes
  - Document bug fixes and documentation updates

Addresses DOC-API-001 and DOC-CHANGELOG from release-15.0.7-documentation.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

CHANGELOG.md

@@ -0,0 +1,56 @@
+# Changelog
+
+All notable changes to cgm-remote-monitor are documented in this file.
+
+## [15.0.7] - 2026-03-XX (Unreleased)
+
+### Added
+
+#### UUID/Identifier Handling (REQ-SYNC-072)
+- **Treatments API**: Server now normalizes client sync identities into a unified `identifier` field
+  - Loop overrides: UUID in `_id` → extracted to `identifier`
+  - Loop carbs/doses: `syncIdentifier` → copied to `identifier`
+  - xDrip+: `uuid` → copied to `identifier`
+  - AAPS: `identifier` unchanged (already correct)
+- **Entries API**: CGM entries with UUID `_id` now handled correctly (GAP-SYNC-045)
+- **Deduplication**: Server uses `identifier` for upsert matching when present
+- **Benefits**: Re-uploading treatments after app reinstall/cache clear updates existing records
+
+#### Test Infrastructure
+- **NODE_ENV=test safety check**: Tests now refuse to run without `NODE_ENV=test`, preventing accidental production database destruction (GAP-SYNC-046)
+- New npm scripts: `test:unit`, `test:integration` for faster development cycles
+- Comprehensive test suite for UUID handling across v1 and v3 APIs
+
+### Changed
+
+#### Node.js Requirements
+- **Minimum**: Node.js 20 LTS (was Node 14/16)
+- **Tested**: Node 20, 22, 24
+- Updated `engines` field in package.json
+
+#### MongoDB Requirements
+- **Minimum**: MongoDB 4.4 (was 4.2)
+- **Tested**: MongoDB 4.4, 5.0, 6.0
+- Improved compatibility with MongoDB 5.x+ `$set` behavior
+
+### Fixed
+
+- **Loop Override Sync** (Issue #8450): Overrides with UUID `_id` no longer cause duplicate records
+- **mmol BG Display**: Fixed OpenAPS tooltips showing incorrect values for mmol/L users
+- **AAPS Entry Dedup**: Added retry logic for CI flakiness in concurrent upload tests
+
+### Documentation
+
+- Updated README.md with current Node.js and MongoDB requirements
+- Added "Running Tests Locally" section to CONTRIBUTING.md
+- Updated treatments schema documentation with identifier normalization behavior
+
+### Security
+
+- Enabled Traditional Chinese localization
+
+---
+
+## [15.0.6] - Previous Release
+
+See GitHub releases for prior changelog entries.