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.

README.md

@@ -0,0 +1,80 @@
+# db-keepalive
+
+Pluggable Go daemon that performs a periodic counter increment to prevent free-tier database clusters (Redis Cloud, Valkey, Aiven, Neon, Supabase, MongoDB Atlas, Couchbase Capella, etc.) from being auto-paused for inactivity.
+
+Successor to the `*-keepalive` family: one binary, one image, six adapters.
+
+## Supported drivers
+
+| `DB_TYPE`    | Driver                              | Env vars |
+| ------------ | ----------------------------------- | -------- |
+| `redis`      | `github.com/redis/go-redis/v9`      | `REDIS_URL` |
+| `valkey`     | `github.com/valkey-io/valkey-go`    | `VALKEY_URL` |
+| `postgresql` | `github.com/lib/pq`                 | `SERVICE_URI` |
+| `mysql`      | `github.com/go-sql-driver/mysql`    | `DATA_SOURCE_NAME` |
+| `mongodb`    | `go.mongodb.org/mongo-driver/v2`    | `MONGODB_URI`, `MONGODB_DATABASE`, `MONGODB_COLLECTION` |
+| `couchbase`  | `github.com/couchbase/gocb/v2`      | `COUCHBASE_CONNECTION_STRING`, `COUCHBASE_USERNAME`, `COUCHBASE_PASSWORD`, `COUCHBASE_BUCKET_NAME`, `COUCHBASE_SCOPE_NAME`, `COUCHBASE_COLLECTION_NAME` |
+
+Optional: `INTERVAL` (e.g. `30s`, `5m`; default `1m`), `COUNTER_KEY` (default `counter`).
+
+## Quick start (Docker)
+
+```bash
+docker run -d --name db-keepalive --restart unless-stopped \
+  -e DB_TYPE=redis \
+  -e REDIS_URL='redis://default@host:6379' \
+  ghcr.io/tiennm99/db-keepalive:latest
+```
+
+## Quick start (local)
+
+```bash
+git clone https://github.com/tiennm99/db-keepalive
+cd db-keepalive
+cp .env.example .env       # then edit DB_TYPE + the driver's env vars
+go run .
+```
+
+## How it works
+
+On every tick the chosen adapter performs the cheapest write that proves the cluster is alive:
+
+- **Redis/Valkey** — `INCR counter`
+- **PostgreSQL** — `UPDATE keepalive SET value = value + 1 WHERE key = 'counter' RETURNING value`
+- **MySQL** — `UPDATE` + `SELECT` inside a transaction
+- **MongoDB** — `FindOneAndUpdate({_id: "counter"}, {$inc: {count: 1}}, upsert)`
+- **Couchbase** — `GET counter` → `++` → `UPSERT counter`
+
+The PostgreSQL and MySQL adapters expect a table:
+
+```sql
+CREATE TABLE keepalive (key TEXT PRIMARY KEY, value BIGINT NOT NULL DEFAULT 0);
+INSERT INTO keepalive (key, value) VALUES ('counter', 0);
+```
+
+(MySQL uses backticked identifiers — see `adapter/mysql.go`.)
+
+## Adding a new database
+
+1. Create `adapter/<name>.go`.
+2. Implement the `Adapter` interface in `adapter/adapter.go` (`Connect`, `Increment`, `Close`).
+3. Register the factory in `init()`:
+   ```go
+   func init() { Registry["<name>"] = func() (Adapter, error) { return &myAdapter{}, nil } }
+   ```
+4. Add an `import _ "your driver"` if needed, and the env vars to `.env.example` and the table above.
+
+## Migrated from
+
+This repo replaces six single-database repos. All are archived with a pointer here:
+
+- [redis-keepalive](https://github.com/tiennm99/redis-keepalive)
+- [valkey-keepalive](https://github.com/tiennm99/valkey-keepalive)
+- [postgresql-keepalive](https://github.com/tiennm99/postgresql-keepalive)
+- [mysql-keepalive](https://github.com/tiennm99/mysql-keepalive)
+- [mongodb-keepalive](https://github.com/tiennm99/mongodb-keepalive)
+- [couchbase-keepalive](https://github.com/tiennm99/couchbase-keepalive)
+
+## License
+
+Apache-2.0 — see [LICENSE](LICENSE).