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).

lib/captain/base_task_service.rb

@@ -32,7 +32,7 @@ class Captain::BaseTaskService
   end
 
   def api_base
-    endpoint = InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_ENDPOINT')&.value.presence || 'https://api.openai.com/'
+    endpoint = Llm::Config.openai_endpoint.presence || 'https://api.openai.com/'
     endpoint = endpoint.chomp('/')
     "#{endpoint}/v1"
   end

lib/integrations/llm_base_service.rb

@@ -84,7 +84,7 @@ class Integrations::LlmBaseService
   end
 
   def api_base
-    endpoint = InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_ENDPOINT')&.value.presence || 'https://api.openai.com/'
+    endpoint = Llm::Config.openai_endpoint.presence || 'https://api.openai.com/'
     endpoint = endpoint.chomp('/')
     "#{endpoint}/v1"
   end

lib/integrations/openai/key_validator.rb

@@ -20,7 +20,7 @@ module Integrations::Openai::KeyValidator
   end
 
   def self.api_base
-    endpoint = InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_ENDPOINT')&.value.presence || 'https://api.openai.com/'
+    endpoint = Llm::Config.openai_endpoint.presence || 'https://api.openai.com/'
     "#{endpoint.chomp('/')}/v1"
   end
 end

lib/llm/config.rb

@@ -29,6 +29,20 @@ module Llm::Config
       yield context
     end
 
+    # Returns the OpenAI-compatible base URL for Captain AI.
+    # - provider == 'custom'  -> use the operator-configured CAPTAIN_OPEN_AI_ENDPOINT
+    # - provider == 'openai'  -> nil (RubyLLM uses the default https://api.openai.com/)
+    def openai_endpoint
+      provider = InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_PROVIDER')&.value.presence || 'openai'
+      return nil unless provider == 'custom'
+
+      InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_ENDPOINT')&.value
+    end
+
+    def system_api_key
+      InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_API_KEY')&.value
+    end
+
     private
 
     def configure_ruby_llm
@@ -39,13 +53,5 @@ module Llm::Config
         config.logger = Rails.logger
       end
     end
-
-    def system_api_key
-      InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_API_KEY')&.value
-    end
-
-    def openai_endpoint
-      InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_ENDPOINT')&.value
-    end
   end
 end