Update model costs (Gemini 3.6/3.7 Flash price cut)

[anchpop]

Summary

• Google cut Gemini 3.6 Flash's standard rate from $1.50/$7.50 to an introductory $0.75/$3.75 per MTok on 2026-08-13/14, to match the newly-launched Gemini 3.7 Flash (added here at the same rate). Both revert to $1.50/$7.50 on 2027-01-01. Flex/priority tiers scaled proportionally per the file's existing convention (flex = 50%, priority = 1.8x of standard).
• Fixed a stale comment on claude-sonnet-5: Anthropic's pricing page confirms the $2/$10 rate is now the permanent standard price — the previously-announced increase to $3/$15 on 2026-09-01 will not happen. The stored rate (2.0/10.0) was already correct; only the comment was wrong.
• Bumped RATE_CARD_VERSION to 2026-08-17.
• Checked Anthropic, OpenAI, and Google pricing against current sources; no other rates in the table have changed since the last update (2026-07-30).

Test plan

• cargo test -p model-prices — all 10 tests pass
• cargo build --workspace — builds clean

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