Add procedural field post-processing to snapshots

server/data_model.py:
- apply_procedural(record) — stub function; add computed fields to the
  proc dict inside and they appear under record['procedural'] in every
  record stored and broadcast.
- _get(data, *keys) — safe nested dict accessor for use inside
  apply_procedural.

collect.py:
- New snapshots: apply_procedural() called after collect_snapshot(),
  before appending to the store and broadcasting.
- Preloaded 24-h history: guarded apply (skip if 'procedural' already
  present) applied after load_last_24h_records().
- Current-day store loaded from disk: same guarded apply after
  load_store(), so legacy records sent to clients are enriched on the fly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/data_model.py

@@ -243,3 +243,39 @@ def collect_snapshot(vehicle, domains: list[str]) -> dict:
         if data is not None:
             snapshot[domain] = data
     return snapshot
+
+
+# ── procedural fields ─────────────────────────────────────────────────────────
+
+def _get(data: dict, *keys):
+    """Navigate nested dicts safely; returns None if any key is missing."""
+    for k in keys:
+        if not isinstance(data, dict):
+            return None
+        data = data.get(k)
+    return data
+
+
+def apply_procedural(record: dict) -> dict:
+    """Compute derived fields and store them under record['procedural'].
+
+    Called for every new snapshot before storage and broadcast, and for
+    historical records that lack the 'procedural' key before they are sent
+    to clients.  Add computed fields to the proc dict below.
+    """
+    proc: dict = {}
+
+    # ── add computed fields here ──────────────────────────────────────────
+    # Use _get(record, "domain", "statusObject", "field", "value") to
+    # safely read any nested value.  Always use {"value": ..., "unit": ...}
+    # format so the GUI picks up the field automatically.
+    #
+    # Example — usable energy estimated from SOC and battery capacity:
+    # soc = _get(record, "charging", "batteryStatus", "currentSOC", "value")
+    # if soc is not None:
+    #     proc["energy_stored"] = {"value": round(soc / 100 * 77.0, 1), "unit": "kWh"}
+    # ─────────────────────────────────────────────────────────────────────
+
+    if proc:
+        record["procedural"] = proc
+    return record