refactor health check status page

[jranson]

this PR overhauls the Health Check Status Page with these improvements:

• Replaces manual JSON body construction w/ a struct->marshal in JSON()
• Extracts Text Output builder to a Tabular() function.
• ^^ effectively separates the data structure construction from its Stringers.
• Supports yaml output via ?yaml url param or Accept: application/yaml
• Adds ALB and Pool health status to the outputs
  • an ALB is considered available if at least 1 pool member is available or unchecked.
• Serves a Last-Modified header and supports 304 responses to If-Modified-Since requests.
• adds updateLock mutex to updateStatusText to ensure it runs in singleton
• Reorders the output order to Unavailable, Available, Unchecked, to ensure the most important section is first

Old Output (Plaintext)

Trickster Backend Health Status            last change: 2025-12-08 07:08:58 UTC
-------------------------------------------------------------------------------

prom1        prometheus   available

prom2        prometheus   unavailable since 2025-12-08 07:08:58 UTC

prom3        prometheus   not configured for automated health checks

-------------------------------------------------------------------------------

New Output (Plaintext)

Trickster Backend Health Status            last change: 2025-12-08 07:08:58 UTC
-------------------------------------------------------------------------------

prom2        prometheus   unavailable since 2025-12-08 07:08:58 UTC

prom1        prometheus   available
alb1         alb (tsm)    available u:[prom2] a:[prom1] nc:[prom3]

prom3        prometheus   not configured for automated health checks

-------------------------------------------------------------------------------

New Output (JSON)

{
  "title": "Trickster Backend Health Status",
  "updateTime": "2025-12-08 07:08:58 UTC",
  "unavailable": [
    {
      "name": "prom2",
      "provider": "prometheus",
      "downSince": "2025-12-08 07:08:58 UTC",
      "detail": "required status code mismatch, got [500] expected one of [200]"
    }
  ],
  "available": [
    {
      "name": "prom1",
      "provider": "prometheus"
    },
    {
      "name": "alb1",
      "provider": "alb",
      "mechanism": "tsm",
      "unavailablePoolMembers": [ "prom2" ],
      "availablePoolMembers": [ "prom1" ],
      "uncheckedPoolMembers": [ "prom3" ]
    }
  ],
  "unchecked": [
    {
      "name": "prom3",
      "provider": "prometheus"
    }
  ]
}

Fixes #876

[coveralls]

Pull Request Test Coverage Report for Build 20023376961

Details

• 2 of 191 (1.05%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.5%) to 73.179%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/daemon/setup/setup.go	0	1	0.0%
pkg/proxy/headers/detect.go	0	3	0.0%
pkg/proxy/handlers/health/health.go	0	185	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/proxy/engines/cache.go	2	92.42%

Totals
Change from base Build 20015258539:	-0.5%
Covered Lines:	14993
Relevant Lines:	20488

---

💛 - Coveralls