feat: complete MiniMax LLM provider integration

Add full MiniMax provider support across the entire stack:

Backend:
- Add MINIMAX to LiteLLMProvider enum in db.py
- Add MINIMAX mapping to all provider_map dicts in llm_service.py,
  llm_router_service.py, and llm_config.py
- Add Alembic migration (rev 106) for PostgreSQL enum
- Add MiniMax M2.5 example in global_llm_config.example.yaml

Frontend:
- Add MiniMax to LLM_PROVIDERS enum with apiBase
- Add MiniMax-M2.5 and MiniMax-M2.5-highspeed to LLM_MODELS
- Add MINIMAX to Zod validation schema
- Add MiniMax SVG icon and wire up in provider-icons

Docs:
- Add MiniMax setup guide in chinese-llm-setup.md

MiniMax uses an OpenAI-compatible API (https://api.minimax.io/v1)
with models supporting up to 204K context window.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

surfsense_backend/alembic/versions/106_add_minimax_to_litellmprovider_enum.py

@@ -0,0 +1,23 @@
+"""Add MINIMAX to LiteLLMProvider enum
+
+Revision ID: 106
+Revises: 105
+"""
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "106"
+down_revision: str | None = "105"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.execute("COMMIT")
+    op.execute("ALTER TYPE litellmprovider ADD VALUE IF NOT EXISTS 'MINIMAX'")
+
+
+def downgrade() -> None:
+    pass