Split src/ into an api/cli/engine/config module hierarchy

- src/api/: HTTP router + DTOs + the MCP server (the HTTP/MCP-facing
  surface): mod.rs (formerly api.rs), dto.rs, mcp_server.rs.
- src/cli/: argument parsing and process entry (formerly inline in
  main.rs) plus session registry: mod.rs, sessions.rs. main.rs is now
  a thin entry point that just declares modules and calls cli::run().
- src/engine/: the actor + everything it depends on to parse/index/
  query log lines: mod.rs (formerly engine.rs), alerts.rs, promql.rs,
  rate.rs, ingest.rs, logfmt.rs, filter_parser.rs/.pest,
  filter_datetime.rs, index_key.rs, data_value.rs, metric_primitive.rs,
  timestamp.rs.
- src/config/: persisted configuration: mod.rs (formerly settings.rs),
  profile.rs.
- src/consts.rs stays at the crate root, used from every module above.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A1ZJ3SrayzLzVndhVq9BmS

src/engine/timestamp.rs

@@ -0,0 +1,24 @@
+use serde::Serialize;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Serialize)]
+pub struct Timestamp(i64);
+
+impl Timestamp {
+    pub fn now() -> Self {
+        let nanos = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .expect("system clock is before the Unix epoch")
+            .as_nanos() as i64;
+        Timestamp(nanos)
+    }
+
+    #[allow(dead_code)]
+    pub fn from_millis(millis: i64) -> Self {
+        Timestamp(millis * 1_000_000)
+    }
+
+    pub fn as_millis(&self) -> i64 {
+        self.0 / 1_000_000
+    }
+}