feat: add API provider selection UI and fix stuck features on agent crash

API Provider Selection:
- Add provider switcher in Settings modal (Claude, Kimi, GLM, Ollama, Custom)
- Auth tokens stored locally only (registry.db), never returned by API
- get_effective_sdk_env() builds provider-specific env vars for agent subprocess
- All chat sessions (spec, expand, assistant) use provider settings
- Backward compatible: defaults to Claude, env vars still work as override

Fix Stuck Features:
- Add _cleanup_stale_features() to process_manager.py
- Reset in_progress features when agent stops, crashes, or fails healthcheck
- Prevents features from being permanently stuck after rate limit crashes
- Uses separate SQLAlchemy engine to avoid session conflicts with subprocess

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/lib/api.ts

@@ -24,6 +24,7 @@ import type {
   Settings,
   SettingsUpdate,
   ModelsResponse,
+  ProvidersResponse,
   DevServerStatusResponse,
   DevServerConfig,
   TerminalInfo,
@@ -399,6 +400,10 @@ export async function getAvailableModels(): Promise<ModelsResponse> {
   return fetchJSON('/settings/models')
 }
 
+export async function getAvailableProviders(): Promise<ProvidersResponse> {
+  return fetchJSON('/settings/providers')
+}
+
 export async function getSettings(): Promise<Settings> {
   return fetchJSON('/settings')
 }

ui/src/lib/types.ts

@@ -525,6 +525,20 @@ export interface ModelsResponse {
   default: string
 }
 
+export interface ProviderInfo {
+  id: string
+  name: string
+  base_url: string | null
+  models: ModelInfo[]
+  default_model: string
+  requires_auth: boolean
+}
+
+export interface ProvidersResponse {
+  providers: ProviderInfo[]
+  current: string
+}
+
 export interface Settings {
   yolo_mode: boolean
   model: string
@@ -533,6 +547,10 @@ export interface Settings {
   testing_agent_ratio: number  // Regression testing agents (0-3)
   playwright_headless: boolean
   batch_size: number  // Features per coding agent batch (1-3)
+  api_provider: string
+  api_base_url: string | null
+  api_has_auth_token: boolean
+  api_model: string | null
 }
 
 export interface SettingsUpdate {
@@ -541,6 +559,10 @@ export interface SettingsUpdate {
   testing_agent_ratio?: number
   playwright_headless?: boolean
   batch_size?: number
+  api_provider?: string
+  api_base_url?: string
+  api_auth_token?: string
+  api_model?: string
 }
 
 export interface ProjectSettingsUpdate {