feat(captain): add OpenAI Custom Server provider option

Add an operator-level provider switch for Captain AI so it can use any
OpenAI-compatible base URL (vLLM, OpenRouter, proxies, etc.) in addition to the
default api.openai.com endpoint.

- installation_config.yml: new CAPTAIN_OPEN_AI_PROVIDER (default 'openai',
  select openai|custom) shown on the Super Admin captain settings page.
- lib/llm/config.rb: openai_endpoint now returns the configured
  CAPTAIN_OPEN_AI_ENDPOINT only when provider=='custom'; nil for openai/default
  so a stale endpoint is ignored. system_api_key/openai_endpoint made public so
  all consumers share one provider-switch source.
- Consumers (base_task_service, llm_base_service, key_validator, ai_agents
  initializer) now route via Llm::Config.openai_endpoint instead of reading
  CAPTAIN_OPEN_AI_ENDPOINT directly.
- installation_config.rb: fail-closed validation (provider must be openai/custom;
  provider=custom requires a present https:// endpoint).

Approved by independent five-key reviews deleg_b141098a (found+fixed one logic
gap) and deleg_3088b922 (passed=true, blocking arrays empty).

config/initializers/ai_agents.rb

@@ -5,7 +5,7 @@ require 'agents'
 Rails.application.config.after_initialize do
   api_key = InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_API_KEY')&.value
   model = InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_MODEL')&.value.presence || LlmConstants::DEFAULT_MODEL
-  api_endpoint = InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_ENDPOINT')&.value || LlmConstants::OPENAI_API_ENDPOINT
+  api_endpoint = Llm::Config.openai_endpoint.presence || LlmConstants::OPENAI_API_ENDPOINT
 
   if api_key.present?
     Agents.configure do |config|

config/installation_config.yml

@@ -191,6 +191,15 @@
   display_title: 'OpenAI API Endpoint (optional)'
   description: 'The OpenAI endpoint configured for use in Captain AI. Default: https://api.openai.com/'
   locked: false
+- name: CAPTAIN_OPEN_AI_PROVIDER
+  display_title: 'OpenAI Provider'
+  description: 'Select the OpenAI-compatible provider: "openai" uses the default api.openai.com endpoint; "custom" lets you set a custom OpenAI-compatible base URL in CAPTAIN_OPEN_AI_ENDPOINT.'
+  locked: false
+  value: openai
+  type: select
+  options:
+    openai: 'OpenAI (default: api.openai.com)'
+    custom: 'OpenAI Custom Server'
 - name: CAPTAIN_EMBEDDING_MODEL
   display_title: 'Embedding Model (optional)'
   description: 'The embedding model configured for use in Captain AI. Default: text-embedding-3-small'