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.

---

Note

Medium Risk
Changes the timeline step calculation used by SelectSeries and exposes it via a new --querier.min-step-duration flag, which can alter query resolution and backend load. Risk is moderate because it affects query behavior across all read paths, though defaults preserve existing behavior.

Overview
Makes the UI timeline/series query step configurable instead of being hard-clamped to 15s by introducing querier.min-step-duration (default 15s) and plumbing it through API registration into the HTTP render handlers.

Adds timeline.CalcPointIntervalWithMinInterval (with guards for sub-second values and rounding not violating the minimum) and switches SelectSeries requests to use it; updates and expands tests to cover custom minimums (e.g., 1s/5s) and edge cases.

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[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.

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[manab-pr]

@simonswine please review it.