feat(providers): add DeepSeek as first-class account provider

Adds DeepSeek as a native Anthropic-compatible provider using its
https://api.deepseek.com/anthropic endpoint, instead of requiring users
to hand-configure it as openai-compatible with manual model mappings.
Wires it through types, CLI, HTTP API, dashboard (including model-mapping
overrides for fable/opus/sonnet/haiku), and pricing.

Addresses item 1 of #434.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>