feat(ai): add first-party MiniMax M3 endpoints

[octo-patch]

Reason: parameter-refresh | 2026-08-11 | MiniMax-M3 is active only through OpenRouter with reasoning effort fixed to medium, while the implementation lacks MiniMax global/CN OpenAI/Anthropic endpoint selection and the target M3 context, pricing/cache, image/video input, and adaptive/disabled thinking parameters; open PRs #115 and #159 add only generic OpenAI-compatible base URLs.

Summary

• Add first-party MiniMax M3 configuration for the global and CN OpenAI- and Anthropic-compatible endpoints.
• Support adaptive or disabled thinking while keeping the existing routed configuration as a fallback.
• Publish the current context window, pricing/cache, and text/image/video input metadata in one tested profile.
• Forward and document the new environment variables for self-hosted deployments.

Checks

• pnpm exec vitest run src/lib/selfhost-preflight.test.ts src/server/lib/openrouter.test.ts
• pnpm exec tsc --noEmit
• pnpm exec oxlint . --type-aware
• pnpm exec prettier --check .env.example .env.selfhost.example compose.yaml package.json pnpm-lock.yaml src/env.d.ts src/lib/selfhost-preflight.ts src/server/features/sam/SamChatAgent.ts src/server/lib/openrouter.ts src/server/lib/openrouter.test.ts src/serverFunctions/samAccess.ts
• git diff --check