feat(settings): add log retention settings page

Sysadmin/admin-only settings page for configuring how long device log
history is kept before pruning. GET/PUT /api/settings/log-retention,
new LogRetentionSettings page, nav entry, and route.

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

backend/settings/log_retention_service.py

@@ -0,0 +1,31 @@
+from shared.firebase import get_db
+from settings.log_retention_models import LogRetentionSettings, LogRetentionSettingsUpdate
+
+COLLECTION = "admin_settings"
+DOC_ID = "log_retention"
+
+
+def get_log_retention() -> LogRetentionSettings:
+    """Get log retention settings from Firestore. Creates defaults if not found."""
+    db = get_db()
+    doc = db.collection(COLLECTION).document(DOC_ID).get()
+    if doc.exists:
+        return LogRetentionSettings(**doc.to_dict())
+    defaults = LogRetentionSettings()
+    db.collection(COLLECTION).document(DOC_ID).set(defaults.model_dump())
+    return defaults
+
+
+def update_log_retention(data: LogRetentionSettingsUpdate) -> LogRetentionSettings:
+    """Update log retention settings. Only provided fields are updated."""
+    db = get_db()
+    doc_ref = db.collection(COLLECTION).document(DOC_ID)
+    doc = doc_ref.get()
+
+    existing = doc.to_dict() if doc.exists else LogRetentionSettings().model_dump()
+    update_data = data.model_dump(exclude_none=True)
+    existing.update(update_data)
+
+    normalized = LogRetentionSettings(**existing)
+    doc_ref.set(normalized.model_dump())
+    return normalized