refactor config to unified provider structure with LLM support

- add Providers map for centralized API key storage
- add Keywords global field for transcription/LLM hints
- add LLMConfig with Enabled, Provider, Model
- add LLMPostProcessingConfig (RemoveStutters, AddPunctuation, FixGrammar, RemoveFillerWords)
- add LLMCustomPromptConfig (Enabled, Prompt)
- add ToLLMConfig() and IsLLMEnabled() methods
- add auto-migration from old transcription.api_key format
- unified API key resolution: providers -> legacy -> env var
- backward compatible with existing configs

tasks/prd.jsonc

@@ -47,7 +47,7 @@
         "Typecheck passes",
         "go test ./internal/config/... passes"
       ],
-      "passes": false
+      "passes": true
     },
     {
       "title": "Create LLM adapter interface and implementations",