feat(web): show available memory in Tools diagnostics

System Diagnostics reported memory as used-percent plus used/total GB.
Neither distinguishes a healthy board from one about to fail, because
page cache counts as used and is reclaimable on demand -- a Pi can read
70% used and be fine, or read the same and be minutes from trouble.

MemAvailable is the kernel's own estimate of what a new allocation can
actually obtain, and it is the number that tracked the failure on a 1GB
Pi 3B+: healthy running sat above 500MB, and the crash came at 73MB. By
that point fork() was failing, so sshd could not spawn a session and
systemd could not respawn the display, while the kernel carried on
answering pings at 0% loss. Used-percent gave no warning at any point on
the way there; available memory fell steadily for hours.

/api/v3/system/status now returns memory_available_mb from
psutil.virtual_memory().available, and Tools renders it as its own tile,
coloured against the thresholds that failure implies: red under 150MB,
amber under 300MB, green above.

The existing memory tile is left alone -- used/total is still what you
want when sizing a workload; this answers the different question of how
much room is left right now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web_interface/blueprints/api_v3.py

@@ -1563,6 +1563,11 @@ def get_system_status():
             'memory_used_percent': round(memory_percent, 1),
             'memory_total_mb': round(memory.total / (1024 * 1024), 1),
             'memory_used_mb': round(memory.used / (1024 * 1024), 1),
+            # MemAvailable, not total-minus-used: it accounts for reclaimable
+            # page cache, so it is what actually predicts memory trouble. A
+            # board can read 70% "used" and be fine, or read the same and be
+            # about to fail fork(), and only this number tells them apart.
+            'memory_available_mb': round(memory.available / (1024 * 1024), 1),
             'cpu_temp': round(cpu_temp, 1) if cpu_temp is not None else None,
             'disk_used_percent': round(disk_percent, 1),
             'disk_total_gb': round(disk.total / (1024 * 1024 * 1024), 1),