feat: add cache token tracking and cache-aware cost calculation

Track cache_read_tokens and cache_write_tokens end-to-end: parse from
provider responses (OpenAI, DeepSeek, Grok, Gemini), persist to SQLite,
apply cache-aware pricing from the model registry, and surface in API
responses and the dashboard.

- Add cache fields to ProviderResponse, StreamUsage, RequestLog structs
- Parse cached_tokens (OpenAI/Grok), prompt_cache_hit/miss (DeepSeek),
  cachedContentTokenCount (Gemini) from provider responses
- Send stream_options.include_usage for streaming; capture real usage
  from final SSE chunk in AggregatingStream
- ALTER TABLE migration for cache_read_tokens/cache_write_tokens columns
- Cache-aware cost formula using registry cache_read/cache_write rates
- Update Provider trait calculate_cost signature across all providers
- Add cache_read_tokens/cache_write_tokens to Usage API response
- Dashboard: cache hit rate card, cache columns in pricing and usage
  tables, cache token aggregation in SQL queries
- Remove API debug panel and verbose console logging from api.js
- Bump static asset cache-bust to v5

src/logging/mod.rs

@@ -16,6 +16,8 @@ pub struct RequestLog {
     pub prompt_tokens: u32,
     pub completion_tokens: u32,
     pub total_tokens: u32,
+    pub cache_read_tokens: u32,
+    pub cache_write_tokens: u32,
     pub cost: f64,
     pub has_images: bool,
     pub status: String, // "success", "error"
@@ -75,8 +77,8 @@ impl RequestLogger {
         sqlx::query(
             r#"
             INSERT INTO llm_requests 
-            (timestamp, client_id, provider, model, prompt_tokens, completion_tokens, total_tokens, cost, has_images, status, error_message, duration_ms, request_body, response_body)
-            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+            (timestamp, client_id, provider, model, prompt_tokens, completion_tokens, total_tokens, cache_read_tokens, cache_write_tokens, cost, has_images, status, error_message, duration_ms, request_body, response_body)
+            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
             "#,
         )
         .bind(log.timestamp)
@@ -86,6 +88,8 @@ impl RequestLogger {
         .bind(log.prompt_tokens as i64)
         .bind(log.completion_tokens as i64)
         .bind(log.total_tokens as i64)
+        .bind(log.cache_read_tokens as i64)
+        .bind(log.cache_write_tokens as i64)
         .bind(log.cost)
         .bind(log.has_images)
         .bind(log.status)