dbdeployer init doesn't support PostgreSQL (hardcoded to MySQL)

[renecannao]

Problem

`dbdeployer init` is hardcoded to download and install a MySQL tarball. There is no way to initialize a PostgreSQL environment, even though dbdeployer v2.x supports PostgreSQL as a first-class provider for `deploy single`, `deploy replication`, etc.

Current behavior

```
$ dbdeployer init --help
Initializes dbdeployer environment:

• creates $SANDBOX_HOME and $SANDBOX_BINARY directories
• downloads and expands the latest MySQL tarball <-- no choice of provider
• installs shell completion file

Flags:
--dry-run Show operations but don't run them
--skip-all-downloads Do not download any file
--skip-shell-completion Do not download shell completion file
--skip-tarball-download Do not download MySQL tarball
```

Code:

```go
// ops/init.go:198
tarball, err := downloads.FindOrGuessTarballByVersionFlavorOS(version, common.MySQLFlavor,
OS, arch, minimal, true, false)
```

The flavor is hardcoded to `common.MySQLFlavor`.

Expected behavior

```
$ dbdeployer init --provider=postgresql

... downloads and installs latest PostgreSQL

```

Proposed fix

1. Add `--provider` flag to `dbdeployer init` (values: `mysql`, `postgresql`, default `mysql`).
2. For `postgresql`, install PostgreSQL via the existing provider path (deb extraction on Linux, similar on macOS).
3. Update help text: "downloads and installs the latest MySQL or PostgreSQL tarball".
4. Add a CI test that exercises `dbdeployer init --provider=postgresql`.

Related

PostgreSQL provider was added in the v2.x rewrite (see PRs on master). `deploy single`, `deploy replication`, and `downloads` all support `--provider=postgresql`. `init` is the remaining hole.

[coderabbitai]

🔗 Related PRs

#44 - Extend Provider interface with lifecycle methods [merged]
#54 - feat: Phase 3 — PostgreSQL provider [merged]
#55 - feat: dbdeployer documentation website (Astro + Starlight) [merged]
#78 - feat: add MariaDB and Percona Server to tarball registry [merged]

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