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.

Dockerfile

@@ -0,0 +1,11 @@
+FROM golang:1.23-alpine AS build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -ldflags="-s -w" -o /out/db-keepalive .
+
+FROM gcr.io/distroless/static-debian12:nonroot
+COPY --from=build /out/db-keepalive /db-keepalive
+USER nonroot:nonroot
+ENTRYPOINT ["/db-keepalive"]