BE: Compare summary endpoint — builds, boots, and tests

[alanpeixinho]

Part of #2009
Depends on #2020 and #2021

GET /api/tree/<tree_name>/<branch>/compare/summary?commit_a=&commit_b=&origin=&filter_*=...

Return filtered comparison summaries for builds, boots, and tests in one response. Detail rows remain in the category-specific compare endpoints.

{
  "builds": {
    "side_a": { "pass": 0, "fail": 0, "inconclusive": 0 },
    "side_b": { "pass": 0, "fail": 0, "inconclusive": 0 },
    "delta": {
      "pass": 0,
      "fail": 0,
      "regressions": 0,
      "fixed": 0,
      "new_failures": 0,
      "new_passes": 0
    },
    "groups": []
  },
  "boots": { "...same shape..." },
  "tests": { "...same shape..." }
}

• side_a / side_b: grouped status counts after filters
• delta.pass / fail: net change (B − A)
• Transitions: PASS→FAIL, FAIL→PASS, null→FAIL, null→PASS; INCONCLUSIVE transitions are excluded
• Identity must match detail comparison:
  • Builds: config + architecture + compiler
  • Boots/tests: path + config + platform + architecture + compiler + lab
• Groups: builds by build identity; boots/tests by path
• Summary only; no detail rows