Cache pip requirements in GitHub Actions

[MilesMorel]

Summary

• Adds cache: 'pip' to actions/setup-python@v5 in both dev.yml and prod.yml, so pip installs reuse a cache keyed on requirements.txt instead of redownloading every run.

Closes #91

Test plan

• Ran act locally against dev.yml — setup-python accepts the input, pip install completes, main.py --debugsources --debugposters runs successfully.
• Confirm on the next push that the "Set up Python" step reports a cache hit on the second run.

🤖 Generated with Claude Code

[binamkayastha]

I don't see the cache being used in the ci check for this repo, maybe we can manually run the dev one to see if it gets picked up? I thought we have to use a special step for this but I could be wrong. Doesn't seem to cause issues so I'm also fine with just merging and testing in prod for this one LGTM!