Refactor webpush store to eliminate code duplication

Consolidate SQLite and Postgres store implementations into a single
commonStore with database-specific SQL queries passed via configuration.
This eliminates ~100 lines of duplicate code while maintaining full
functionality for both backends.

Key changes:
- Move all store methods to commonStore in store.go
- Remove sqliteStore and postgresStore wrapper structs
- Refactor SQLite to use QueryRow() pattern like Postgres
- Pass database-specific queries via storeQueries struct
- Make store types unexported, only expose Store interface

All tests pass for both SQLite and PostgreSQL backends.

server/server.go

@@ -58,7 +58,7 @@ type Server struct {
 	messagesHistory   []int64                             // Last n values of the messages counter, used to determine rate
 	userManager       *user.Manager                       // Might be nil!
 	messageCache      *messageCache                       // Database that stores the messages
-	webPush           webpush.Store                        // Database that stores web push subscriptions
+	webPush           webpush.Store                       // Database that stores web push subscriptions
 	fileCache         *fileCache                          // File system based cache that stores attachments
 	stripe            stripeAPI                           // Stripe API, can be replaced with a mock
 	priceCache        *util.LookupCache[map[string]int64] // Stripe price ID -> price as cents (USD implied!)