feat: period-scoped holiday management, archive lifecycle, and docs/release sync

- add period-scoped holiday architecture end-to-end
	- model: scope `SchoolHoliday` to `academic_period_id`
	- migrations: add holiday-period scoping, academic-period archive lifecycle, and merge migration head
	- API: extend holidays with manual CRUD, period validation, duplicate prevention, and overlap merge/conflict handling
	- recurrence: regenerate holiday exceptions using period-scoped holiday sets

- improve frontend settings and holiday workflows
	- bind holiday import/list/manual CRUD to selected academic period
	- show detailed import outcomes (inserted/updated/merged/skipped/conflicts)
	- fix file-picker UX (visible selected filename)
	- align settings controls/dialogs with defined frontend design rules
	- scope appointments/dashboard holiday loading to active period
	- add shared date formatting utility

- strengthen academic period lifecycle handling
	- add archive/restore/delete flow and backend validations/blocker checks
	- extend API client support for lifecycle operations

- release/docs updates and cleanup
	- bump user-facing version to `2026.1.0-alpha.15` with new changelog entry
	- add tech changelog entry for alpha.15 backend changes
	- refactor README to concise index and archive historical implementation docs
	- fix Copilot instruction link diagnostics via local `.github` design-rules reference

docs/archive/CLEANUP_SUMMARY.md

@@ -0,0 +1,39 @@
+# Database Cleanup Summary
+
+## Files Removed ✅
+
+The following obsolete database initialization files have been removed:
+
+### Removed Files:
+- **`server/init_database.py`** - Manual table creation (superseded by Alembic migrations)
+- **`server/init_db.py`** - Alternative initialization (superseded by `init_defaults.py`)  
+- **`server/init_mariadb.py`** - Database/user creation (handled by Docker Compose)
+- **`server/test_sql.py`** - Outdated connection test (used localhost instead of container)
+
+### Why These Were Safe to Remove:
+1. **No references found** in any Docker files, scripts, or code
+2. **Functionality replaced** by modern Alembic-based approach
+3. **Hardcoded connection strings** that don't match current Docker setup
+4. **Manual processes** now automated in production deployment
+
+## Current Database Management ✅
+
+### Active Scripts:
+- **`server/initialize_database.py`** - Complete initialization (NEW)
+- **`server/init_defaults.py`** - Default data creation
+- **`server/init_academic_periods.py`** - Academic periods setup
+- **`alembic/`** - Schema migrations (version control)
+
+### Development Scripts (Kept):
+- **`server/dummy_clients.py`** - Test client data generation
+- **`server/dummy_events.py`** - Test event data generation
+- **`server/sync_existing_clients.py`** - MQTT synchronization utility
+
+## Result
+
+- **4 obsolete files removed**
+- **Documentation updated** to reflect current state
+- **No breaking changes** - all functionality preserved
+- **Cleaner codebase** with single initialization path
+
+The database initialization process is now streamlined and uses only modern, maintained approaches.