Server health status: add Waybar-friendly output format

[MattHandzel]

Problem

Current server health status output is not optimized for compact status bars like Waybar.

Goal

Add a health status format that is trivial to consume from Waybar custom modules.

Proposal

Expose one stable, machine-readable endpoint/command output (JSON preferred), plus optional plain text.

Option A (preferred)

• Add endpoint: GET /health/waybar
• Return compact JSON object with fixed keys:
  • text (short display string)
  • tooltip (expanded diagnostics string)
  • class (status class; e.g. ok, degraded, down)
  • percentage (0-100 overall health score)

Example:

{
  "text": "Lifelog: OK",
  "tooltip": "server=up db=up collector=laptop:connected ingest=flowing lag=3s",
  "class": "ok",
  "percentage": 98
}

Option B

• Add CLI mode: lifelog-server-backend --health-waybar with identical JSON shape.

Requirements

• Output must be stable (no key churn) for script consumers.
• Response time should be fast (<200ms local path).
• Must include at least:
  • server availability
  • DB connectivity
  • ingest flow status (recent chunks received window)
  • collector connectivity summary

Acceptance Criteria

• Waybar custom module can parse and display status with no custom post-processing beyond jq -r.
• During failure states, class and text clearly indicate degraded/down.
• Document usage in USAGE.md with a Waybar config snippet.