feat: [BE] Add page_size param to pipeline runs list endpoint

[Mbeaulne]

Summary

Add a page_size query parameter to GET /api/pipeline_runs/ so clients can control how many runs are returned per page.

Motivation

The Dashboard homepage dynamically sizes its "My Runs" and "All Runs" tables based on available screen height. Currently the backend always returns a fixed number of runs per page, so the frontend has to over-fetch and slice client-side — which is wasteful and means pagination skips items.

Proposed Change

Add an optional page_size integer query parameter to GET /api/pipeline_runs/:

• Default: current behavior (no change when omitted) 10
• Minimum: 1
• Maximum: suggested cap of 100

Related

• Part of epic: epic: Homepage Redesign tangle-ui#1964

[yuechao-qin]

@Ark-kun and I were just talking about this the other day. My local tests appear to support this well in performance. I think it's worth us testing this in staging. @Ark-kun thoughts?