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/app/config/global_llm_config.example.yaml

@@ -183,6 +183,23 @@ global_llm_configs:
     use_default_system_instructions: true
     citations_enabled: true
 
+  # Example: MiniMax M2.5 - High-performance with 204K context window
+  - id: -8
+    name: "Global MiniMax M2.5"
+    description: "MiniMax M2.5 with 204K context window and competitive pricing"
+    provider: "MINIMAX"
+    model_name: "MiniMax-M2.5"
+    api_key: "your-minimax-api-key-here"
+    api_base: "https://api.minimax.io/v1"
+    rpm: 60
+    tpm: 100000
+    litellm_params:
+      temperature: 1.0  # MiniMax requires temperature in (0.0, 1.0], cannot be 0
+      max_tokens: 4000
+    system_instructions: ""
+    use_default_system_instructions: true
+    citations_enabled: true
+
 # =============================================================================
 # Image Generation Configuration
 # =============================================================================