feat: Add database import/export functionality (Issue #10)

Backend:
- New /api/v1/backup/export endpoint - exports all data as single JSON
- New /api/v1/backup/import endpoint - imports JSON with merge or replace
- Handles vendors, filaments, spools, adjustments, settings
- Remaps IDs to maintain relationships during import

Frontend:
- Added Backup & Restore section to Settings page
- Download Backup button for export
- Import with merge or replace options
- Warning confirmation for replace mode

This enables migrating between PostgreSQL and SQLite databases.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

spoolman/api/v1/router.py

@@ -15,7 +15,7 @@ from spoolman.database.database import backup_global_db
 from spoolman.exceptions import ItemNotFoundError
 from spoolman.ws import websocket_manager
 
-from . import export, externaldb, field, filament, models, other, setting, spool, vendor
+from . import backup as backup_module, export, externaldb, field, filament, models, other, setting, spool, vendor
 
 logger = logging.getLogger(__name__)
 
@@ -112,3 +112,4 @@ app.include_router(field.router)
 app.include_router(other.router)
 app.include_router(externaldb.router)
 app.include_router(export.router)
+app.include_router(backup_module.router)