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>

test/test_system_status_available_memory.py

@@ -0,0 +1,92 @@
+"""/api/v3/system/status must report MemAvailable, not just used/total.
+
+"Memory used %" cannot tell a healthy board from one about to fail. Page cache
+counts as used and is reclaimable on demand, so a Pi can read 70% used and be
+perfectly 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 at
+500MB+, the crash happened at 73MB, and by then fork() was failing -- sshd
+could not spawn a session and systemd could not respawn the display, while the
+kernel carried on answering pings.
+
+psutil.virtual_memory().available is MemAvailable on Linux. total - used is not
+a substitute: they diverge exactly when unreclaimable memory (shmem, tmpfs) is
+in play, which is when the distinction matters.
+"""
+
+import json
+import sys
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+from flask import Flask
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+MB = 1024 * 1024
+
+
+@pytest.fixture
+def client():
+    pytest.importorskip("psutil")
+    app = Flask(__name__)
+    app.config["TESTING"] = True
+    from web_interface.blueprints.api_v3 import api_v3
+    for attr in ("config_manager", "plugin_manager", "cache_manager"):
+        setattr(api_v3, attr, MagicMock())
+    if "api_v3" not in app.blueprints:
+        app.register_blueprint(api_v3, url_prefix="/api/v3")
+    return app.test_client()
+
+
+def _memory(total_mb, used_mb, available_mb):
+    m = MagicMock()
+    m.total = total_mb * MB
+    m.used = used_mb * MB
+    m.available = available_mb * MB
+    m.percent = round(used_mb / total_mb * 100, 1)
+    return m
+
+
+def _get_status(client, memory):
+    # The endpoint caches for 10s; bypass so each case is measured fresh.
+    with patch("web_interface.cache.get_cached", return_value=None), \
+         patch("psutil.virtual_memory", return_value=memory), \
+         patch("psutil.cpu_percent", return_value=5.0), \
+         patch("psutil.boot_time", return_value=0.0):
+        resp = client.get("/api/v3/system/status")
+    assert resp.status_code == 200, resp.data
+    return json.loads(resp.data)["data"]
+
+
+def test_available_memory_is_reported(client):
+    data = _get_status(client, _memory(total_mb=905, used_mb=620, available_mb=284))
+    assert "memory_available_mb" in data
+    assert data["memory_available_mb"] == pytest.approx(284, abs=0.5)
+
+
+def test_available_is_not_total_minus_used(client):
+    # The case the readout exists for: 600MB is "not used", but only 300MB can
+    # actually be allocated. Reporting used% alone would call this healthy.
+    data = _get_status(client, _memory(total_mb=1000, used_mb=400, available_mb=300))
+
+    derived = data["memory_total_mb"] - data["memory_used_mb"]
+    assert derived == pytest.approx(600, abs=1)
+    assert data["memory_available_mb"] == pytest.approx(300, abs=0.5)
+    assert data["memory_available_mb"] != pytest.approx(derived, abs=1), \
+        "available must come from MemAvailable, not be derived from used"
+
+
+def test_existing_memory_fields_are_unchanged(client):
+    data = _get_status(client, _memory(total_mb=905, used_mb=620, available_mb=284))
+    assert data["memory_total_mb"] == pytest.approx(905, abs=0.5)
+    assert data["memory_used_mb"] == pytest.approx(620, abs=0.5)
+    assert "memory_used_percent" in data
+
+
+def test_a_nearly_exhausted_board_reports_a_small_number(client):
+    # 73MB available is what the board actually read when it stopped being able
+    # to fork. The readout has to surface that rather than round it away.
+    data = _get_status(client, _memory(total_mb=905, used_mb=800, available_mb=73))
+    assert data["memory_available_mb"] == pytest.approx(73, abs=0.5)