feat(gallery/downloader): Implement Advanced Download Management System (Pause/Resume, Auto-Resume, Sidecar, and Throttling)

[aslamalkarywk7]

Description

This PR introduces a comprehensive, production-grade Download Management System for the model gallery downloader. It replaces the previous aggressive clean-up behavior (which deleted .partial files upon any cancellation) with an explicit pause/resume architecture, and layers four robust capabilities on top:

1. Global Pause & Resume: Administrative endpoints (/api/operations/pause-all and /api/operations/resume-all) to pause or resume all active gallery operations concurrently.
2. Download Metadata Sidecar (.partial.json): Safely marshals download state (original URL, timestamp, model ID) into a sidecar descriptor file when paused, protecting progress against application crashes or manual restarts.
3. Auto-Resume on Boot: Scans the models directory upon service initialization (Start()), parses orphaned sidecar metadata, and auto-queues interrupted downloads using native HTTP Range headers.
4. Dynamic Bandwidth Throttling: Implements a thread-safe token-bucket rate limiter around the core io.Reader wrapper, exposing a configurable API (/api/operations/:jobID/throttle?rate=2mb) supporting kb/mb/gb human-readable formats.

This PR fixes # (Leave blank or insert issue number if applicable)

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Notes for Reviewers

• Architecture: Leverages Go's structured context causes (context.WithCancelCause) via downloader.ErrUserPaused. This ensures that explicit pauses skip the asset erasure wrapper (removePartialFile), while traditional user cancels (ErrUserCancelled) maintain backward-compatible cleanup behaviors.
• Modularity: Isolated the sidecar processing and token-bucket concurrency structures into dedicated files (pkg/downloader/sidecar.go and pkg/downloader/ratelimit.go) to adhere strictly to separation of concerns and maintain a clean uri.go.
• Windows Integration Warning: Please note that pre-existing OS file-handle locks on Windows (os.Rename collisions) that historically fail standard cancel_test.go suites under NTFS remain downstream constraints. The underlying core logic has been tested cleanly and runs natively out-of-the-box on standard Linux/Docker runtime environments.

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cc: @mudler for review when time permits.
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