Fix video streaming Range handling, Windows paths and player wiring

[pranitdhanade-sys]

Motivation

• Browser playback and seeking were failing due to incomplete/incorrect handling of Range headers and inconsistent response headers/status codes.
• There was no reliable local-file fallback and absolute path handling was fragile on Windows.
• The frontend used a hardcoded demo URL instead of the backend stream endpoint, preventing real playback testing.

Description

• Add a robust parseRangeHeader(rangeHeader, fileSize) that supports open-ended and suffix byte ranges and returns validated start/end or { invalid: true } for RFC-compliant handling.
• Introduce streamFromLocalFile(req, res, localFilePath, fallbackContentType) to asynchronously stat and stream local files (Windows-safe path resolution via path.resolve/path.normalize), with proper headers and 206/200/416 responses and cleanup on client disconnect.
• Harden GridFS streaming: validate ranges with the new parser, set Accept-Ranges, Content-Length, Content-Type, and conditional Content-Range, return 206 for partial responses and 200 for full-file responses, and ensure downloadStream.destroy() is called when the request closes.
• Replace LOCAL_VIDEOS_DIR with path.resolve(...), avoid blocking statSync calls by using fs.promises.stat, and normalize absolute local paths.
• Update the frontend Frontend/videoplayer-live.html to fetch the playlist from /api/videos and set the <video> src to video.src (or fallback to /api/videos/:id/stream) instead of a hardcoded external demo file.

Testing

• Ran a static syntax check with node --check Backend/routes/videos.js, which succeeded.
• Performed a code review and diff inspection to verify header logic, range parsing, stream cleanup, and frontend wiring; no automated unit/integration tests were present for the streaming endpoint in this repo.
• Recommend runtime validation (playback + seeking in browsers and testing with large files and malformed Range headers) in staging to confirm end-to-end behavior.

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Codex Task