Sidecar: auth always-on, protocol version, rich health

Auth is now impossible to turn off by accident. A blank auth_token is never
allowed even on loopback: config load generates a token, writes it back into
sidecar.toml (preserving the rest of the file), logs it, and continues -- so a
forgotten or cleared token self-heals into a working, authenticated setup instead
of silently disabling auth.

  No auth token configured.
  Generated new token: cb99...
  Saved to sidecar.toml. Authentication is on.

Protocol versioning (PROTOCOL_VERSION = 1) lets the website and sidecar detect a
mismatch immediately when a message shape changes. Every response carries an
X-UOLink-Version header; /health and ws.hello include "protocol"; a request that
declares a different X-UOLink-Version is rejected 409 with both versions so the
mismatch is unambiguous. Bump the constant when a contract changes.

/health is now a real troubleshooting panel: status (ok/degraded), protocol,
plugin_connected (is the shard link up), database (SELECT 1), uptime, and
last_event (the timestamp of the last line from the shard). Unauthenticated so
monitoring can reach it.

Verified: a blank token generates + persists + enforces (401 without, 200 with);
X-UOLink-Version header on every response; 409 on a declared mismatch; /health
reports degraded/plugin_connected:false with no shard, then flips to ok/true and a
populated last_event once the shard connects.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sidecar/src/store.rs

@@ -34,6 +34,12 @@ impl Store {
         Ok(Self { pool })
     }
 
+    /// Cheap liveness check for the health endpoint.
+    pub async fn ping(&self) -> anyhow::Result<()> {
+        sqlx::query("SELECT 1").execute(&self.pool).await?;
+        Ok(())
+    }
+
     /// Appends one live event. Failures are logged by the caller; persistence must never block the
     /// live feed.
     pub async fn insert_event(&self, t: i64, kind: &str, json: &str) -> anyhow::Result<()> {