监控页面增强：温度变化闪绿光、观测N分钟前、更新戳可见

- 每张卡片加"观测 X 分钟前"时间感知
- 温度值变化时卡片边框闪绿光（HTMX afterSwap 事件）
- 更新时间戳移入 HTMX 局部刷新区，30s 刷新可见
- CSS 加 obs-age 样式

monitoring-web/src/db.rs

@@ -5,6 +5,7 @@ pub struct ObsRow {
     pub temp_c: Option<f64>,
     #[allow(dead_code)]
     pub obs_time: Option<String>,
+    pub created_at: Option<String>,
 }
 
 /// Get recent temperature observations for an ICAO station.
@@ -14,7 +15,7 @@ pub fn get_recent_obs(db_path: &str, icao: &str, minutes: i32, limit: usize) ->
         Err(_) => return vec![],
     };
     let sql = format!(
-        "SELECT temp_c, obs_time FROM airport_obs_log \
+        "SELECT temp_c, obs_time, created_at FROM airport_obs_log \
          WHERE icao = ?1 AND created_at > datetime('now', '{} minutes') \
          ORDER BY created_at DESC LIMIT ?2",
         -minutes
@@ -28,6 +29,7 @@ pub fn get_recent_obs(db_path: &str, icao: &str, minutes: i32, limit: usize) ->
             Ok(ObsRow {
                 temp_c: row.get(0)?,
                 obs_time: row.get(1)?,
+                created_at: row.get(2)?,
             })
         })
         .ok()

monitoring-web/src/main.rs

@@ -59,6 +59,18 @@ fn load_city_snapshot(db_path: &str, (key, display, icao, airport, tz, thresh):
     let temps: Vec<f64> = obs.iter().filter_map(|o| o.temp_c).collect();
     let current_temp = temps.first().copied();
 
+    // Age of most recent observation
+    let obs_age_min = obs.first().and_then(|o| {
+        o.created_at.as_ref().and_then(|ts| {
+            chrono::NaiveDateTime::parse_from_str(ts, "%Y-%m-%d %H:%M:%S")
+                .ok()
+                .map(|dt| {
+                    let obs_utc = dt.and_utc();
+                    (now_utc - obs_utc).num_minutes().max(0)
+                })
+        })
+    });
+
     // Trend from last 6 points
     let trend_data: Vec<f64> = temps.iter().take(6).copied().collect();
     let trend = trend::calc_trend(&trend_data);
@@ -94,6 +106,7 @@ fn load_city_snapshot(db_path: &str, (key, display, icao, airport, tz, thresh):
         temp_ok: false,
         trend_ok: false,
         in_window: false,
+        obs_age_min,
     }
 }
 

monitoring-web/src/model.rs

@@ -19,6 +19,7 @@ pub struct CitySnapshot {
     pub temp_ok: bool,
     pub trend_ok: bool,
     pub in_window: bool,
+    pub obs_age_min: Option<i64>,
 }
 
 #[derive(Debug, Clone, Copy, Serialize)]