Add Spring Boot-style actuator endpoints (/manage/*)

[Grizaster]

Overview

Adds Spring Boot-style management endpoints under /manage/ for operational monitoring of the HTTP server. The existing /health endpoint is kept for backwards compatibility.

Endpoints

Endpoint	Description
GET /manage/health	Health check with real disk space via statvfs (Unix)
GET /manage/info	Build info: app name, version, architecture, OS, timezone, language
GET /manage/env	Filtered environment variables — secrets (TOKEN, KEY, PASSWORD, SECRET, CREDENTIAL) are excluded
GET /manage/metrics	Render metrics: total/PDF/failed counts, average render time, uptime

Implementation Details

• Metrics tracked via AtomicU64 counters and Instant timer
• failed_renders incremented on all error paths (render, PNG encode, image decode, PDF encode)
• /manage/health reports real disk space values via statvfs (Unix) with 10MB threshold
• /manage/env filters sensitive variables by name pattern
• /manage/metrics reports avg_ms correctly (no erroneous unit conversion)

Dependencies

• serde_json (optional, behind serve feature) for structured JSON responses
• libc (Unix only, target-specific) for statvfs in /manage/health

Files Changed

• src/main.rs — actuator endpoints, metrics tracking, shared app state
• Cargo.toml — serde_json dependency + libc for Unix targets
• Cargo.lock — updated

[Copilot]

Pull request overview

This PR adds Spring Boot-style “actuator” management endpoints under /manage/* to the serve (Axum HTTP server) mode, while keeping the legacy /health endpoint for backward compatibility.

Changes:

• Adds /manage/health, /manage/info, /manage/env, and /manage/metrics endpoints and introduces shared in-memory metrics state.
• Tracks basic render/request metrics using atomic counters and reports uptime.
• Extends dependencies to support JSON responses (serde_json) and Unix disk stats via statvfs (libc).

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 6 comments.

File	Description
src/main.rs	Adds management endpoints, shared AppState, disk-space health reporting, and render/request metrics collection.
Cargo.toml	Adds serde_json for JSON responses and libc (Unix) for statvfs; updates serve feature deps.
Cargo.lock	Locks new transitive dependencies from serde_json and libc.

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