fix: only write analysis jpegs for alarm frames

[SteveGilvarry]

Fixes #4996

Problem

Since the 1.38 analysis pipeline rewrite, packet->analysis_image is created for every frame that goes through motion detection (so the live Analysis view shows full colour instead of greyscale). Event::AddFrame wrote an analysis jpeg whenever the packet had one — which is now always — so SaveJPEGs = Analysis images only (if available) saved every analysed frame of the event instead of only alarmed frames as in 1.36. On a 10fps Always Recording + Always Analysing monitor that is ~6,000 jpegs (~4GB) per 10-minute event vs ~400MB on 1.36.

Fix

Gate the analysis-jpeg write on frame_type == ALARM, restoring 1.36 semantics: analysis jpegs are only written for alarm frames (including scored pre/post-alarm buffer frames). The live analysis view is unaffected.

🤖 Generated with Claude Code

[connortechnology]

I think the if (packet->analysis_image) used to do the gating. But now we always assign analysis_image. Instead we should only populate analysis-image if we have motion detection score (See line 2364 of zm_monitor.cpp).

Thing is, AI comes later and may also create it, but with a zero motion score, so frame type != ALARM.

So I don't think this is the way to go.