Videos: Add log-statement to convert_video_avc.go

[Akashic101]

Just a small change, this prints out that no conversion is needed when a video already had been previously converted. Currently running docker compose exec photoprism photoprism convert is a very quiet command with basically no output if already run at a previous point, leaving me unsure how much progress has been achieved. This small log fixes that by providing a quick log about where the converter is right now

Acceptance Criteria:

• Features and enhancements must be fully implemented so that they can be released at any time without additional work
• Automated unit and/or acceptance tests are mandatory to ensure the changes work as expected and to reduce repetitive manual work
• Frontend components must be responsive to work and look properly on phones, tablets, and desktop computers; you must have tested them on all major browsers and different devices
• Documentation and translation updates should be provided if needed
• In case you submit database-related changes, they must be tested and compatible with SQLite 3 and MariaDB 10.5.12+

[CLAassistant]

All committers have signed the CLA.

[lastzero]

@Akashic101 My apologies for not getting back to you any sooner!

We should be able to merge this if you change the log message as follows:

convert: %s is AVC encoded, no transcoding required

[lastzero]

@Akashic101 I'm happy to report that your proposal has been implemented as part of the above commit. I will therefore close this PR as it is no longer needed. Thank you for your contribution! 👍