refactor: provider & models (#277)

* refactor: move client_type to provider, replace model fields with config JSONB

- Move `client_type` from `models` to `llm_providers` table
- Add `icon` field to `llm_providers`
- Replace `dimensions`, `input_modalities`, `supports_reasoning` on `models`
  with a single `config` JSONB column containing `dimensions`,
  `compatibilities` (vision, tool-call, image-output, reasoning),
  and `context_window`
- Auto-imported models default to vision + tool-call + reasoning
- Update all backend consumers (agent, flow resolver, handlers, memory)
- Regenerate sqlc, swagger, and TypeScript SDK
- Update frontend forms, display, and i18n for new schema

* ui: show provider icon avatar in sidebar and detail header, remove icon input

* feat: add built-in provider registry with YAML definitions and enable toggle

- Add `enable` column to llm_providers (default true, backward-compatible)
- Create internal/registry package to load YAML provider/model definitions
  on startup and upsert into database (new providers disabled by default)
- Add conf/providers/ with OpenAI, Anthropic, Google YAML definitions
- Add RegistryConfig to TOML config (providers_dir, default conf/providers)
- Model listing APIs and conversation flow now filter by enabled providers
- Frontend: enable switch in provider form, green status dot in sidebar,
  enabled providers sorted to top

* fix: make 0041 migration idempotent for fresh databases

Guard data migration steps with column-existence checks so the
migration succeeds on databases created from the updated init schema.

internal/memory/adapters/builtin/dense_runtime.go

@@ -525,14 +525,20 @@ func resolveDenseEmbeddingModel(ctx context.Context, queries *dbsqlc.Queries, mo
 	if err != nil {
 		return denseModelSpec{}, fmt.Errorf("dense runtime: get embedding provider: %w", err)
 	}
-	if !row.Dimensions.Valid || row.Dimensions.Int32 <= 0 {
+	var cfg struct {
+		Dimensions *int `json:"dimensions"`
+	}
+	if len(row.Config) > 0 {
+		_ = json.Unmarshal(row.Config, &cfg)
+	}
+	if cfg.Dimensions == nil || *cfg.Dimensions <= 0 {
 		return denseModelSpec{}, fmt.Errorf("dense runtime: embedding model %s missing dimensions", modelRef)
 	}
 	return denseModelSpec{
 		modelID:    strings.TrimSpace(row.ModelID),
 		baseURL:    strings.TrimSpace(provider.BaseUrl),
 		apiKey:     strings.TrimSpace(provider.ApiKey),
-		dimensions: int(row.Dimensions.Int32),
+		dimensions: *cfg.Dimensions,
 	}, nil
 }