unify JIRA epic collector to v3 API; build: add CONTAINER_CMD/PLATFORM vars for cross-arch builds

[base698]

⚠️ Pre Checklist

Please complete ALL items in this checklist, and remove before submitting

• I have read through the Contributing Documentation.
• I have added relevant tests.
• I have added relevant documentation.
• I will add labels to the PR, such as pr-type/bug-fix, pr-type/feature-development, etc.

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Summary

This PR improves container build flexibility and unifies the JIRA epic collector to use the v3 /search/jql endpoint exclusively.

Key changes:

• Makefiles
  • Introduces CONTAINER_CMD (defaults to docker) and PLATFORM (defaults to linux/amd64) for flexible builds across environments.
  • Updates all docker build commands to respect $(CONTAINER_CMD) and --platform=$(PLATFORM).
• Backend Dockerfile
  • Adds -Wno-incompatible-pointer-types and USE_BUNDLED_ZLIB=ON to CMake flags for stable cross-compile of libgit2.
  • Simplifies make install -j invocation.
• JIRA Epic Collector
  • Removes api/2/search (Server/DC) collector path.
  • Always uses the api/3/search/jql endpoint.
  • Inlines next-page handling via nextPageToken and removes redundant v2/v3 helper functions.

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Does this close any open issues?

• [Bug][Jira] V2 API removed #8563 still broken for people according to the comments.

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Screenshots

N/A

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Other Information

Motivation

• Jira Collection broken against atlassian.net

[klesh]

Hi, we need to support Jira Server 7, which doesn't support api v3, removing api v2 is not acceptable.

[base698]

Closing in lieu of: #8612