feat: add admin HTTP server, MQTT schema discovery, CLI Protobuf decoding, and float display

[electricalgorithm]

Overview

Introduces a dedicated HTTP Admin Server to manage the ProtoMQ broker at runtime. This allows for live management without polluting the core MQTT hot-paths or relying on complex MQTT-based authentication for administrative tasks.

Key Features

• Conditionally Compiled: The Admin Server is strictly optional and disabled by default. It can be built into the server via the zig build -Dadmin_server=true flag.
• Zero Overhead Footprint: When the build flag is disabled, the binary retains its exact bare-metal memory footprint (verified at ~2.62 MB for 100 concurrent clients), ensuring embedded targets aren't affected.
• Shared Event Loop: To avoid complex mutex locking, the HTTP listener runs as a non-blocking TCP socket cooperatively on the exact same IOContext event loop as the MQTT server.
• Bearer Token Auth: All endpoints are secured by checking the Authorization: Bearer <token> header against the ADMIN_TOKEN environment variable.

Endpoints Implemented

• POST /api/v1/schemas: Dynamically registers a new .proto schema file to memory and persists it to the ./schemas/ directory, mapping it to a specified MQTT topic.
• GET /api/v1/schemas: Retrieves a JSON map of all currently active topics and their linked Protobuf message types.
• GET /metrics: Telemetry endpoint returning a JSON payload of:
  • Active TCP connections
  • Total routed messages across the broker
  • Total loaded schemas

Testing & Automation

• Integration Tests: Added tests/cases/admin_server_test.sh to the main run_all.sh CI loop. This suite boots the server, tests 401 Unauthorized rejections, registers a schema, and pulls metrics.
• Benchmark Fixes: Patched the Python benchmark suite (pyproject.toml) to correctly resolve the psutil dependency required for tracking performance footprints natively.
• Documentation: Updated README.md to detail the new Admin Server parameters.

[electricalgorithm]

I'm deploying the changes to my server and investigating the functionality.

[electricalgorithm]

This PR was originally scoped to the HTTP Admin Server, but has since grown to include a full end-to-end feature chain: remote deployment, service management, CLI schema discovery, and Protobuf decoding improvements. All features are motivated by the goal of making ProtoMQ a self-describing, operationally friendly MQTT broker.

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What's Included

1. 🛠️ HTTP Admin Server (src/server/admin.zig)

A lightweight, optional HTTP server that runs alongside the MQTT broker and exposes runtime management endpoints.

Method	Path	Description
GET	/metrics	Returns connection count, messages routed, schema count, and memory usage
GET	/api/v1/schemas	Lists all registered Protobuf schemas and their topic mappings
POST	/api/v1/schemas	Registers a new topic → message type mapping at runtime

Opt-in via -Dadmin=true build flag — adds zero overhead in the default build.

2. 🔍 MQTT Service Discovery

The broker now handles a reserved system topic pair for schema advertisement. Publishing to $SYS/discovery/request triggers a ServiceDiscoveryResponse Protobuf reply on $SYS/discovery/response containing all known topic→schema mappings and the embedded .proto source for each type. This allows any client to become schema-aware with no out-of-band configuration.

3. 📟 CLI Schema Discovery & Auto-Decoding (src/mqtt_cli.zig)

protomq-cli subscribe now automatically decodes Protobuf messages:

# Auto-decode via Service Discovery
protomq-cli subscribe -t sensor/data --proto-dir schemas --host 192.168.178.31

# Direct decode — useful when the broker doesn't advertise the topic
protomq-cli subscribe -t sensor/temp --proto-dir schemas --type SensorData --host 192.168.178.31