release: pre-release detection substring-matches, misclassifying stable tags

## Problem

`.github/workflows/release.yml` decides whether a tag is a pre-release with substring matching:

```bash
if [[ "$TAG" == *"rc"* ]] || [[ "$TAG" == *"beta"* ]] || [[ "$TAG" == *"alpha"* ]] || [[ "$TAG" == *"dev"* ]]; then
```

Any tag *containing* those letters anywhere matches, not just as a version suffix. Verified:

| Tag | Classified | Correct? |
|---|---|---|
| `v2.2.0` | stable | yes |
| `v2.2.0-rc1` | prerelease | yes |
| `v2.2.0-arcadia` | prerelease | **no** — "a**rc**adia" |
| `v2.2.0-devil` | prerelease | **no** — "**dev**il" |

## Why it matters more now

This was cosmetic when it only set the GitHub Release `prerelease` flag. The `publish-image` job now consumes the same flag (via the `release` job's `is_prerelease` output) to gate whether a tag moves the `latest` container image tag:

```yaml
type=raw,value=latest,enable=${{ needs.release.outputs.is_prerelease == 'false' }}
```

A false-positive prerelease means a genuine stable release **silently does not publish `latest`** — and `docker/compose.example.yml` pins `ghcr.io/get2knowio/remo-web:latest` by default, so home-lab users would just never receive that release. It fails quietly: the build succeeds, version tags publish, only `latest` is missing.

## Suggested fix

Match a PEP 440 / SemVer pre-release *suffix* rather than any substring, e.g.:

```bash
if [[ "$TAG" =~ -(rc|beta|alpha|dev)[0-9]*$ ]]; then
```

## Likelihood

Low — it needs an unusual tag name. Filing because the blast radius grew when image publishing started depending on it, and the failure is silent.

Found while adding the GHCR publish job (PR #51).

Start with .github/workflows/release.yml and inspect how TAG is classified and exposed through the release job output. Check the listed stable and prerelease examples, including names containing rc, beta, alpha, or dev, and confirm stable releases still enable the latest image while valid suffixes remain prereleases.

Written by the indexing model from the issue text.