Add Prometheus metrics, JSON logs and provider health checks

- GET /metrics (bearer server.metrics_token): build info, users, sessions,
  pending requests, per-model request/token/cost counters, provider_up
- CODECRAFT_LOG_FORMAT=json for one JSON object per log line
- Providers are probed every 5 minutes via their model list; status shown
  in the provider settings with a manual re-check

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/store.rs

@@ -964,6 +964,25 @@ impl Store {
 // --- audit log & backup ----------------------------------------------------------
 
 impl Store {
+    // --- metrics ---------------------------------------------------------------
+
+    /// All-time usage per provider/model: (provider, model, requests, errors, in, out, cache_read, cache_write, cost, duration_ms).
+    pub fn usage_totals(&self) -> Result<Vec<(String, String, i64, i64, i64, i64, i64, i64, f64, i64)>> {
+        let conn = self.conn.lock().unwrap();
+        let mut stmt = conn.prepare(
+            "SELECT provider, model, COUNT(*), COALESCE(SUM(error),0), COALESCE(SUM(input_tokens),0), COALESCE(SUM(output_tokens),0),
+                    COALESCE(SUM(cache_read_tokens),0), COALESCE(SUM(cache_write_tokens),0), COALESCE(SUM(cost),0), COALESCE(SUM(duration_ms),0)
+             FROM usage GROUP BY provider, model",
+        )?;
+        let rows = stmt.query_map([], |r| Ok((r.get(0)?, r.get(1)?, r.get(2)?, r.get(3)?, r.get(4)?, r.get(5)?, r.get(6)?, r.get(7)?, r.get(8)?, r.get(9)?)))?;
+        Ok(rows.collect::<std::result::Result<_, _>>()?)
+    }
+
+    pub fn count(&self, table: &str) -> i64 {
+        // Table names are fixed strings from the code, never user input.
+        self.conn.lock().unwrap().query_row(&format!("SELECT COUNT(*) FROM {table}"), [], |r| r.get(0)).unwrap_or(0)
+    }
+
     // --- scheduled jobs -------------------------------------------------------
 
     fn job_row(r: &rusqlite::Row) -> rusqlite::Result<crate::jobs::Job> {