database: create shared database package with configurable patterns

Extract ~200 lines of duplicate database connection code from api/ntpdb/
and monitor/ntpdb/ into common/database/ package. Creates foundation for
database consolidation while maintaining zero breaking changes.

Files added:
- config.go: Unified configuration with package-specific defaults
- connector.go: Dynamic connector pattern from Boostport
- pool.go: Configurable connection pool management
- metrics.go: Optional Prometheus metrics integration
- interfaces.go: Shared database interfaces for consistent patterns

Key features:
- Configuration-driven approach (API: 25/10 connections + metrics,
  Monitor: 10/5 connections, no metrics)
- Optional Prometheus metrics when registerer provided
- Backward compatibility via convenience functions
- Flexible config file loading (explicit paths + search-based)

Dependencies: Added mysql driver and yaml parsing for database configuration.

go.mod

@@ -4,10 +4,12 @@ go 1.23.5
 
 require (
 	github.com/abh/certman v0.4.0
+	github.com/go-sql-driver/mysql v1.9.3
 	github.com/labstack/echo-contrib v0.17.2
 	github.com/labstack/echo/v4 v4.13.3
 	github.com/oklog/ulid/v2 v2.1.0
 	github.com/prometheus/client_golang v1.20.5
+	github.com/prometheus/client_model v0.6.1
 	github.com/remychantenay/slog-otel v1.3.2
 	github.com/samber/slog-echo v1.14.8
 	github.com/samber/slog-multi v1.2.4
@@ -28,9 +30,11 @@ require (
 	golang.org/x/net v0.33.0
 	golang.org/x/sync v0.10.0
 	google.golang.org/grpc v1.69.2
+	gopkg.in/yaml.v3 v3.0.1
 )
 
 require (
+	filippo.io/edwards25519 v1.1.0 // indirect
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cenkalti/backoff/v4 v4.3.0 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
@@ -47,7 +51,6 @@ require (
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pierrec/lz4/v4 v4.1.22 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
-	github.com/prometheus/client_model v0.6.1 // indirect
 	github.com/prometheus/common v0.61.0 // indirect
 	github.com/prometheus/procfs v0.15.1 // indirect
 	github.com/samber/lo v1.47.0 // indirect