feat(workers): Concurrent task processing with resource-based backoff

Workers can now process multiple tasks concurrently (default: 3 max).
Self-regulate based on resource usage - back off at 85% memory or 90% CPU.

Backend changes:
- TaskWorker handles concurrent tasks using async Maps
- Resource monitoring (memory %, CPU %) with backoff logic
- Heartbeat reports active_task_count, max_concurrent_tasks, resource stats
- Decommission support via worker_commands table

Frontend changes:
- Workers Dashboard shows tasks per worker (N/M format)
- Resource badges with color-coded thresholds
- Pod visualization with clickable selection
- Decommission controls per worker

New env vars:
- MAX_CONCURRENT_TASKS (default: 3)
- MEMORY_BACKOFF_THRESHOLD (default: 0.85)
- CPU_BACKOFF_THRESHOLD (default: 0.90)
- BACKOFF_DURATION_MS (default: 10000)

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

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

backend/migrations/074_worker_commands.sql

@@ -0,0 +1,27 @@
+-- Migration: Worker Commands Table
+-- Purpose: Store commands for workers (decommission, etc.)
+-- Workers poll this table after each task to check for commands
+
+CREATE TABLE IF NOT EXISTS worker_commands (
+  id SERIAL PRIMARY KEY,
+  worker_id TEXT NOT NULL,
+  command TEXT NOT NULL,  -- 'decommission', 'pause', 'resume'
+  reason TEXT,
+  issued_by TEXT,
+  issued_at TIMESTAMPTZ DEFAULT NOW(),
+  acknowledged_at TIMESTAMPTZ,
+  executed_at TIMESTAMPTZ,
+  status TEXT DEFAULT 'pending'  -- 'pending', 'acknowledged', 'executed', 'cancelled'
+);
+
+-- Index for worker lookups
+CREATE INDEX IF NOT EXISTS idx_worker_commands_worker_id ON worker_commands(worker_id);
+CREATE INDEX IF NOT EXISTS idx_worker_commands_pending ON worker_commands(worker_id, status) WHERE status = 'pending';
+
+-- Add decommission_requested column to worker_registry for quick checks
+ALTER TABLE worker_registry ADD COLUMN IF NOT EXISTS decommission_requested BOOLEAN DEFAULT FALSE;
+ALTER TABLE worker_registry ADD COLUMN IF NOT EXISTS decommission_reason TEXT;
+ALTER TABLE worker_registry ADD COLUMN IF NOT EXISTS decommission_requested_at TIMESTAMPTZ;
+
+-- Comment
+COMMENT ON TABLE worker_commands IS 'Commands issued to workers (decommission after task, pause, etc.)';