feat: Add bandwidth & speed scheduler with timetable and metered connection support

[ja7ad]

Preflight checklist

• I searched existing issues and this has not been proposed already.
• I checked the README and this is not already supported.

Component

Core Engine / Desktop GUI / Configuration

Problem

1. Network Congestion During Active Hours: Hydra's multi-connection engine aggressively consumes available bandwidth, which saturates shared home/office networks during working hours or peak gaming/streaming times.
2. Manual Intervention Overhead: Users currently have to manually pause downloads or manually tweak global speed limits when starting/stopping work.
3. Metered Connection Hazards: When switching to mobile hotspots or metered connections, unconstrained background downloads can rapidly deplete limited data quotas.

Proposed solution

Implement a dynamic Bandwidth & Speed Scheduler that programmatically adjusts download rates, connection limits, and active queues based on time windows and network status.

Key Capabilities

1. Configurable Time-Window Rules (Config / CLI):

• Support a timetable configuration in config.toml (or daemon configuration) specifying time blocks, days of the week, and respective rate limits:
• Normal Speed Limit: Default speed ceiling during general usage.
• Alternative / Off-Peak Speed Limit: Unrestricted or high speeds during off-peak hours (e.g., 12:00 AM – 07:00 AM).
• Work/Strict Limit: Restrictive cap during business hours (e.g., Mon–Fri, 09:00 AM – 06:00 PM: 2 MB/s).

2. GUI Weekly Timetable Grid:

• Provide an interactive 7x24 weekly matrix view in Options > Speed / Scheduler (similar to qBittorrent/Transmission) where users can paint hourly slots with:
• Full Speed
• Alternative Speed Limit
• Paused / Queue Only

3. Live Rate Limiter Adaptation (Zero Disconnection):

• The underlying token-bucket rate limiter in the core engine should dynamically update its refill capacity upon schedule transitions without terminating active multi-part TCP streams or resetting chunk buffers.

4. Metered Network Detection & Guard:

• Detect OS metered connection states (via NetworkInformation on Windows, NetworkManager metered property on Linux, or SCNetworkReachability flags on macOS) to auto-pause or throttle downloads unless explicitly bypassed.

Alternatives considered

• Static Global Speed Limit: Requires manual switching twice a day.
• External OS-Level Traffic Control (tc, NetLimiter, pf): Hard to configure, platform-dependent, and outside Hydra's direct control.

Platforms this should cover

• Linux
• macOS
• Windows
• Android / iOS (via libhydra)

Additional context

Proposed Configuration Schema (config.toml)

[speed_limit]
global_download_rate = "0" # 0 = unlimited

[scheduler]
enabled = true
alt_download_rate = "2MB/s"
alt_upload_rate = "500KB/s"

# Active alternative speed windows
[[scheduler.rules]]
days = ["Mon", "Tue", "Wed", "Thu", "Fri"]
start_time = "09:00"
end_time = "18:00"
action = "alt_limit" # "alt_limit" | "pause" | "unlimited"

[[scheduler.rules]]
days = ["Everyday"]
start_time = "00:00"
end_time = "07:00"
action = "unlimited"

Compatibility

• This changes existing behavior (flags, defaults, output format, or the C ABI).

Contribution

• I'm willing to open a pull request implementing this.