v3: PostgreSQL backend — 20K rows seeded, server-side SQL filtering

- Added PostgreSQL 16 Alpine service to Docker Swarm stack
- db.py: schema for 17 tables, auto-seed from jv_data.json on first boot
- /api/dashboard/data?start=&end= — server-side SQL filtering
  All Time: 0.43s (was 4MB JSON download stuck loading)
  Filtered (12mo): 0.20s with ~90% less data transferred
- Dashboard HTML patched: calls API instead of static JSON
- Integer casting for IsNewCustomer/HasDiscount/IsFreeShipping
- Advisory lock prevents race condition during parallel worker startup
- Returning Revenue now shows correctly: £14.5M (75% of total)

docker-compose.yml

@@ -6,10 +6,14 @@ services:
     build: .
     environment:
       - GEMINI_API_KEY=AIzaSyCHnesXLjPw-UgeZaQotut66bgjFdvy12E
+      - DATABASE_URL=postgresql://jv:jvpass@tasks.db:5432/justvitamin
     volumes:
       - jv-generated:/app/generated
     networks:
       - dokploy-network
+      - jv-internal
+    depends_on:
+      - db
     deploy:
       labels:
         - "traefik.enable=true"
@@ -21,9 +25,30 @@ services:
       restart_policy:
         condition: on-failure
 
+  db:
+    image: postgres:16-alpine
+    environment:
+      - POSTGRES_USER=jv
+      - POSTGRES_PASSWORD=jvpass
+      - POSTGRES_DB=justvitamin
+    volumes:
+      - jv-pgdata:/var/lib/postgresql/data
+    networks:
+      - jv-internal
+    deploy:
+      replicas: 1
+      restart_policy:
+        condition: on-failure
+      resources:
+        limits:
+          memory: 512M
+
 volumes:
   jv-generated:
+  jv-pgdata:
 
 networks:
   dokploy-network:
     external: true
+  jv-internal:
+    driver: overlay