fix: add provider warmup to prevent cold-start timeout on first channel message

The first API request after daemon startup consistently timed out (120s)
when using channels (Telegram, Discord, etc.), requiring a retry before
succeeding. This happened because the reqwest HTTP client's connection
pool was cold — no TLS handshake, DNS resolution, or HTTP/2 negotiation
had occurred yet.

The fix adds a `warmup()` method to the Provider trait that establishes
the connection pool on startup by hitting a lightweight endpoint
(`/api/v1/auth/key` for OpenRouter). The channel server calls this
immediately after creating the provider, before entering the message
processing loop.

Tested on Raspberry Pi 5 (aarch64) with OpenRouter + DeepSeek v3.2 via
Telegram channel. Before: first message took 2-7 minutes (120s timeout +
retries). After: first message responds in <30s with no retries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/providers/traits.rs

@@ -14,4 +14,10 @@ pub trait Provider: Send + Sync {
         model: &str,
         temperature: f64,
     ) -> anyhow::Result<String>;
+
+    /// Warm up the HTTP connection pool (TLS handshake, DNS, HTTP/2 setup).
+    /// Default implementation is a no-op; providers with HTTP clients should override.
+    async fn warmup(&self) -> anyhow::Result<()> {
+        Ok(())
+    }
 }