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/providers/deepseek.rs

@@ -90,12 +90,16 @@ impl super::Provider for DeepSeekProvider {
         model: &str,
         prompt_tokens: u32,
         completion_tokens: u32,
+        cache_read_tokens: u32,
+        cache_write_tokens: u32,
         registry: &crate::models::registry::ModelRegistry,
     ) -> f64 {
         helpers::calculate_cost_with_registry(
             model,
             prompt_tokens,
             completion_tokens,
+            cache_read_tokens,
+            cache_write_tokens,
             registry,
             &self.pricing,
             0.14,