chore: improve docstring coverage across codebase

[biodrone]

Context

CodeRabbit flagged docstring coverage at 42.31% on PR #551 — threshold is 80%.

Scope

Audit Go and Python source files and add missing docstrings to exported functions, types, and methods. Focus on:

• streamdl.go — main entry point and tick loop
• download_stream.go — download functions and helpers
• grpc_client.go — gRPC client functions
• vod_db.go — VOD database operations
• config.go / config_reader.go — config types and parsing
• move_file.go — file move helpers
• streamdl_proto_srv.py — Python gRPC server

Guidelines

• Document the why, not just the what
• Keep docstrings concise — one or two sentences is fine for most functions
• Don't add docstrings to test files or generated code

[biodrone]

Fixed in #559 — docstring coverage now at 91.67%.