ci: build and push the spotify service image

[mattDev0]

The deploy for f80fc13 failed:

WARNING: Some service image(s) must be built from source by running:
    docker compose build spotify-service
Error response from daemon: error from registry: denied

Cause

docker-compose.prod.yml references ghcr.io/mattdev0/devops-control-center/devops-spotify, but the build job only ever built the agent, orchestrator and frontend. That image was never pushed, so the pull was denied.

My omission in #8 — I added the service to compose and to the prod override, but not to the build matrix. The deploy was the first thing to notice.

Fix

• Build, push and Trivy-scan devops-spotify alongside the other three.
• Its Dockerfile lint and cargo test added to both deploy-compose.yml and test.yml, so a PR would catch this class of thing before main.

Tests

The workflow now runs cargo test for the service, which had none. Added six covering the storage layer, focused on the property the poller depends on:

• re-inserting an identical play is a no-op — recently-played returns overlapping windows on every poll, so without this the counts inflate steadily
• the same track at a new timestamp counts again
• discovery correctly splits distinct tracks from repeats
• genre weighting carries an artist's play count to each of their genres
• hour and weekday buckets emit every slot, so the chart axis stays stable as data arrives

cargo test    6 passed
cargo clippy --all-targets -- -D warnings    clean

Note

This must merge before #8 can deploy successfully — or merge both, since the build job is what #8's deploy needs.