feat: initial db-keepalive — pluggable adapters for 6 free-tier DBs

Consolidates the *-keepalive family (redis, valkey, postgresql, mysql,
mongodb, couchbase) into a single binary with a runtime DB_TYPE dispatch
and an Adapter interface. Original repos archived with a pointer here.

adapter/postgresql.go

@@ -0,0 +1,49 @@
+package adapter
+
+import (
+	"context"
+	"database/sql"
+
+	_ "github.com/lib/pq"
+)
+
+func init() {
+	Registry["postgresql"] = func() (Adapter, error) { return &postgresAdapter{}, nil }
+	Registry["postgres"] = Registry["postgresql"]
+}
+
+type postgresAdapter struct {
+	db *sql.DB
+}
+
+func (a *postgresAdapter) Connect(ctx context.Context) error {
+	uri, err := envOrFail("SERVICE_URI")
+	if err != nil {
+		return err
+	}
+	db, err := sql.Open("postgres", uri)
+	if err != nil {
+		return err
+	}
+	a.db = db
+	return a.db.PingContext(ctx)
+}
+
+func (a *postgresAdapter) Increment(ctx context.Context) (int64, error) {
+	tx, err := a.db.BeginTx(ctx, &sql.TxOptions{Isolation: sql.LevelReadCommitted})
+	if err != nil {
+		return 0, err
+	}
+	var value int64
+	if err := tx.QueryRowContext(ctx,
+		`UPDATE keepalive SET value = value + 1 WHERE key = 'counter' RETURNING value`,
+	).Scan(&value); err != nil {
+		tx.Rollback()
+		return 0, err
+	}
+	return value, tx.Commit()
+}
+
+func (a *postgresAdapter) Close(_ context.Context) error {
+	return a.db.Close()
+}