Add backend contributor issue and PR templates

[TommoHCIO]

Summary

• Adds a backend-focused GitHub issue form for VeilLend API/indexer/auth/admin work.
• Adds a repository pull request template with backend impact, verification, documentation, and security/privacy checks.
• Keeps the change scoped to contributor process templates requested by Add issue and pull request templates for backend contributors #29.

Closes #29

Verification

• git diff --check — passed.
• python YAML parse of .github/ISSUE_TEMPLATE/backend_task.yml — passed (yaml-ok, 8 body items).
• Local Markdown/path link check for the new templates — passed (missing_links []).

Notes

• Template-only change; no backend runtime files or package locks were modified.
• No secrets, credentials, wallet keys, hidden prompts, or private runtime instructions are included.