feat: enable prompt caching for Anthropic and OpenAI providers (#104)

* feat: enable prompt caching for Anthropic and OpenAI providers

Activate automatic prompt caching on both LLM providers to reduce input
token costs on multi-turn conversations. Anthropic gets cache_control:
ephemeral (90% savings on cache hits), OpenAI GPT-5.x gets 24h extended
cache retention (free). Cache metrics flow end-to-end through the entire
data pipeline: provider → session → server SSE → MQ protocol → storage →
Prometheus metrics → admin Usage tab.

- AnthropicProvider: top-level cache_control on all requests, extract
  cache_creation_input_tokens and cache_read_input_tokens from streaming
  and non-streaming responses
- OpenAIProvider: prompt_cache_retention=24h for GPT-5.x, extract
  cached_tokens from usage.prompt_tokens_details
- UsageInfo: new cache_creation_tokens and cache_read_tokens fields
- Migration 020: add cache columns to usage_events table
- Storage: record_usage_event and query_usage updated (sqlite + pg)
- Metrics: turnstone_tokens_total{type="cache_creation|cache_read"}
- Server: on_status passes cache tokens to SSE, storage, and metrics
- MQ: StatusEvent carries cache fields through bridge
- SDKs: Python and TypeScript StatusEvent types updated
- OpenAPI: UsageBreakdownItem schema includes cache fields
- Console UI: Usage tab shows cache write/read as secondary readouts
  with visual separator, dimmed when zero
- 16 new tests, docs and 3 diagrams updated

* fix: address Copilot review feedback

- Fix MQ protocol diagram clipping by switching to vertical package
  layout (inbound on top, outbound below) with package aliases
- Regenerate OpenAPI snapshots to include cache_creation_tokens and
  cache_read_tokens on UsageBreakdownItem
- Replace fragile MagicMock(spec=[]) + del pattern with
  types.SimpleNamespace in cache metrics missing-attributes test

docs/diagrams/png/03-core-engine-classes.png

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