fixed torrentstreamming setting Last-Modified header wrong

[CortexQuack]

Summary

This fixes vlc mobile not playing episodes or completly crashing, the original code was setting the Last-Modified http header to the current time, which is not the expected thing to do, by giving time.Time{} it will set the header to nothing

note:

• i am not a go developer, i do know to code
• i think the same issue can be found in internal/directstream/stream_helpers.go however im not sure how to play a normal stream on the external player and it plays normally on the website/app
• there was no issue on github about this but i found complains on the discord server and its a 1 line change so i hope it doesnt go too far from CONTRIBUTING.md
• i only tested it through running the already existing tests and manually checking some torrents before and after the change, i have not checked if this affects other parts of the code

AI Disclosure

• Tool(s) used: mainly Claude Sonnet 5
• What AI was used for: discovering the issue through the vlc logs and finding the relevant go api for the change

Checklist

• I have read CONTRIBUTING.md
• I set up Seanime locally.
• I understand the code I changed.
• I tested the change locally.
• I wrote this PR description myself.
• I disclosed any AI assistance.
• I can explain and maintain this change.