fix(identity): Use unique session IDs for proxy rotation + add task pool gate

- Fix buildEvomiProxyUrl to use passed session ID from identity pool
  instead of truncating to worker+region (causing same IP for all workers)
- Add task pool gate feature with database-backed state
- Add /tasks/pool/toggle endpoint and UI toggle button
- Fix isTaskPoolPaused() missing await in claimTask

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/migrations/111_system_settings.sql

@@ -0,0 +1,35 @@
+-- Migration: 111_system_settings.sql
+-- Description: System settings table for runtime configuration
+-- Created: 2024-12-14
+
+CREATE TABLE IF NOT EXISTS system_settings (
+  key VARCHAR(100) PRIMARY KEY,
+  value TEXT NOT NULL,
+  description TEXT,
+  updated_at TIMESTAMPTZ DEFAULT NOW(),
+  updated_by INTEGER REFERENCES users(id)
+);
+
+-- Task pool gate - controls whether workers can claim tasks
+INSERT INTO system_settings (key, value, description) VALUES
+  ('task_pool_open', 'true', 'When false, workers cannot claim new tasks from the pool')
+ON CONFLICT (key) DO NOTHING;
+
+-- Updated at trigger
+CREATE OR REPLACE FUNCTION update_system_settings_updated_at()
+RETURNS TRIGGER AS $$
+BEGIN
+  NEW.updated_at = NOW();
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS system_settings_updated_at ON system_settings;
+CREATE TRIGGER system_settings_updated_at
+  BEFORE UPDATE ON system_settings
+  FOR EACH ROW
+  EXECUTE FUNCTION update_system_settings_updated_at();
+
+COMMENT ON TABLE system_settings IS 'Runtime configuration settings';
+COMMENT ON COLUMN system_settings.key IS 'Setting name (e.g., task_pool_open)';
+COMMENT ON COLUMN system_settings.value IS 'Setting value as string';