Add supports_thinking field to AIModel to flag models that support
Anthropic's adaptive thinking API (Sonnet 4.6+, Opus 4.7+).

When supports_thinking=True, _anthropic_factory automatically passes
thinking={'type': 'adaptive'} to ChatAnthropic. This enables the model
to reason internally before producing its response, improving accuracy
on complex multi-step tasks without any changes required at call sites.

Migration 0007 adds the column (idempotent — handles pre-existing column
via information_schema check) and seeds True for current thinking models.

Migration 0008 updates the model catalog to the May 2026 lineup:
- Deactivates claude-haiku-3-5, claude-sonnet-4, claude-sonnet-4-5
- Adds claude-sonnet-4-6 and claude-opus-4-7 with supports_thinking=True
- Updates SiteAIConfig defaults to Haiku 4.5 (free) / Sonnet 4.6 (paid)