chore(ci): align image tag conventions with faktenforum

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Summary

Aligns this repo's image tag conventions with faktenforum/faktenforum so prod compose stacks can use the same IMAGE_TAG=latest-dev / IMAGE_TAG=latest semantics across all services.

Before

Trigger	Tags pushed
Push to main	latest
Push to feature branch	<branch-name>, dev

After (matches faktenforum's deploy-dev.yml + release.yml)

Trigger	Tags pushed
Push to main	<short-commit-hash>, latest-dev
Release tag v*	<version> (e.g. v1.2.3), latest
workflow_dispatch on main	<short-commit-hash>, latest-dev

Motivation

Tried to deploy the search stack in Portainer:

Failed to deploy a stack: compose up operation failed:
Error response from daemon: manifest unknown

The ff-dev-* stacks use IMAGE_TAG=latest-dev, but this repo never published a :latest-dev tag. Picking a different tag in the search stack alone would have been a one-off exception across an otherwise uniform set of dev stacks; aligning the tagging is the smaller and longer-lived fix.

Migration

After merge:

1. The first push to main produces ghcr.io/faktenforum/checkbot-rag:latest-dev and :<commit-hash>.
2. Portainer ff-dev-search stack can keep IMAGE_TAG=latest-dev.
3. Older tags (:latest, :dev, branch tags) remain in GHCR until manually pruned - no breakage.

Feature branch builds

Removed for cost/noise reasons. If you need an image from a feature branch:

• Go to Actions -> Build and Push Docker Image -> Run workflow -> pick the branch
• Result: ghcr.io/faktenforum/checkbot-rag:<short-commit-hash> (no latest-dev from non-main branches)

Test plan

• Merge -> watch CI build latest-dev + commit-hash tags
• Pull ghcr.io/faktenforum/checkbot-rag:latest-dev locally to verify
• Portainer ff-dev-search deploys without manifest unknown