feat: Add working hours for natural traffic patterns

Workers check their timezone (from preflight IP geolocation) and current
hour's weight probability to determine availability. This creates natural
traffic patterns - more workers active during peak hours, fewer during
off-peak. Tasks queue up at night and drain during the day.

Migrations:
- 099: working_hours table with hourly weights by profile
- 100: Add timezone column to worker_registry
- 101: Store timezone from preflight IP geolocation
- 102: check_working_hours() function with probability roll

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

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

backend/migrations/100_worker_timezone.sql

@@ -0,0 +1,19 @@
+-- Migration: 100_worker_timezone.sql
+-- Description: Add timezone column to worker_registry for working hours support
+-- Created: 2024-12-13
+
+-- Add timezone column to worker_registry
+-- Populated from preflight IP geolocation (e.g., 'America/New_York')
+ALTER TABLE worker_registry
+ADD COLUMN IF NOT EXISTS timezone VARCHAR(50);
+
+-- Add working_hours_id to link worker to a specific working hours profile
+-- NULL means use default 'natural_traffic' profile
+ALTER TABLE worker_registry
+ADD COLUMN IF NOT EXISTS working_hours_id INTEGER REFERENCES working_hours(id);
+
+-- Index for workers by timezone (useful for capacity planning)
+CREATE INDEX IF NOT EXISTS idx_worker_registry_timezone ON worker_registry(timezone);
+
+COMMENT ON COLUMN worker_registry.timezone IS 'IANA timezone from preflight IP geolocation (e.g., America/New_York)';
+COMMENT ON COLUMN worker_registry.working_hours_id IS 'Reference to working_hours profile. NULL uses default natural_traffic.';