ci: always build backend image on v* tags regardless of changed paths

[restlessankyyy]

What changed

Removes the paths: filter from the push trigger of the Backend Image
workflow so that every v* tag push always builds and publishes its
container image. The paths: filter is kept on the pull_request trigger.

Why

GitHub applies a push paths: filter to tag pushes as well as branch pushes.
Because the release flow (release.yml) tags whatever commit is on main, a
release whose commit only touched infra/ was silently skipped by Backend
Image and never published its versioned image. That is exactly what happened
with v0.2.0: the release and tag exist, but no :0.2.0 image was pushed to
GHCR.

With this change:

• Every release tag reliably builds and pushes its image (type=semver
  metadata maps v0.2.1 to the 0.2.1 image tag).
• Pushes to main also always build, even infra-only commits. This is harmless
  because the deployment image tag is content-addressed (a hash of the backend
  build inputs), so an unchanged backend just republishes the same digest.
• Pull requests stay path-filtered, so PR noise is unchanged (this PR still runs
  Backend Image build-only because it edits the workflow file).

Architecture impact

None. This only changes a CI trigger filter. No application code, component,
data flow, or deployment topology changes, so docs/ARCHITECTURE.md needs no
update.

Testing

• Backend Image runs on this PR (build-only, no push) to validate the workflow.
• The fix takes effect on the next v* tag pushed by the release workflow.