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.

.gitignore

@@ -0,0 +1,7 @@
+.env
+*.pem
+ca.pem
+/db-keepalive
+/dist/
+*.test
+*.out