fix: honest costs in streams, oracle-sized timeouts, history & log cleanup

Cost showed $0 for every streamed request: the X-Cortiq-Cost-USD header is
written before generation, when the cost cannot be known. The gateway now
ships the final number in a closing SSE event (`cortiq_cost_usd`, alongside
the token estimate when the provider sent no usage), and the console prefers
it over the header. Money renders with 4 decimals, the last two in a smaller
font — $0.0006 stays readable instead of collapsing to $0.

Timeouts, both ends: the router default was 800 ms, silently degrading every
prompt the hosted router escalates to its oracle (~10 s) — now 15 s. Provider
clients had NO timeout at all (a dead OpenRouter host hung requests forever) —
now a 10 s connect timeout, with no overall cap so long generations and
streams are never cut.

Benchmark history: per-run × and a Clear-all button. Logs view: a clear
button backed by DELETE /admin/api/logs.

src/config.rs

@@ -305,7 +305,9 @@ fn default_router_enabled() -> bool {
     true
 }
 fn default_router_timeout() -> u64 {
-    800
+    // the hosted router escalates hard prompts to an oracle (~10 s);
+    // an 800 ms budget silently degraded every one of those to the fallback
+    15_000
 }
 fn default_verify_tls() -> bool {
     true
@@ -736,7 +738,7 @@ impl Default for Config {
             router: RouterCfg {
                 enabled: true,
                 url: "https://138.226.222.209".to_string(),
-                timeout_ms: 12_000, // hard prompts escalate to an oracle (~10 s)
+                timeout_ms: 15_000, // hard prompts escalate to an oracle (~10 s)
                 verify_tls: false,  // the hosted endpoint is IP-bound
                 api_key_env: Some("CORTIQ_ROUTER_KEY".to_string()),
                 taxonomy_id: Some("data-assistant".to_string()),