Fix/configurable min step duration

[manab-pr]

     Fixes #4524
    fix: make minimum step duration configurable for eBPF profiling

     This change addresses issue #4524 where users setting `collect_interval`
     below 15 seconds (e.g., 5s for eBPF profiling) were seeing their data
     displayed at 15-second intervals in the UI due to a hardcoded minimum
     step duration.

     Changes:
     - Added `--querier.min-step-duration` flag (default: 15s) to allow users to
       configure the minimum step duration for timeline calculations
     - Created `CalcPointIntervalWithMinInterval` function that accepts a custom
       minimum interval parameter
     - Updated HTTP handlers to pass the configurable value through the call chain
     - Added comprehensive tests covering eBPF use cases (1s, 5s intervals)
     - Maintains backward compatibility with 15-second default

     Users can now run Pyroscope with `--querier.min-step-duration=5s` to support
     fast eBPF profiling collection intervals while maintaining fine-grained
     resolution in the UI.

[simonswine]

@manab-pr do you mind rebasing this? This seems to contain also the changeset from #4526 .

[manab-pr]

@simonswine Sorry about that! I accidentally included changes from #4526 when I started
working on this issue. I've now rebased the branch and it should be clean - only
contains the eBPF profiling changes for #4524. Could you please take another look
when you get a chance?

[manab-pr]

@simonswine The branch has been rebased and is ready for another look. Could you please review when you get a chance? Thanks.