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

static/js/dashboard.js

@@ -348,7 +348,7 @@ class Dashboard {
                 <div class="table-container">
                     <table class="table" id="usage-table">
                         <thead>
-                            <tr><th>Date</th><th>Client</th><th>Provider</th><th>Model</th><th>Requests</th><th>Tokens</th><th>Cost</th></tr>
+                            <tr><th>Date</th><th>Client</th><th>Provider</th><th>Model</th><th>Requests</th><th>Tokens</th><th>Cache Read</th><th>Cache Write</th><th>Cost</th></tr>
                         </thead>
                         <tbody></tbody>
                     </table>
@@ -383,7 +383,7 @@ class Dashboard {
                 <div class="table-container">
                     <table class="table" id="pricing-table">
                         <thead>
-                            <tr><th>Provider</th><th>Model</th><th>Input Cost</th><th>Output Cost</th><th>Last Updated</th></tr>
+                            <tr><th>Provider</th><th>Model</th><th>Input Cost</th><th>Output Cost</th><th>Cache Read</th><th>Cache Write</th></tr>
                         </thead>
                         <tbody></tbody>
                     </table>