feat: implement horizontal scaling optimizations for authz service

- Add /health and /ready endpoints for load balancer health checks
- Replace in-memory JWT token cache with Redis for multi-replica support
- Reduce DB connection pool from 100 to 25 connections per replica
- Add distributed rate limiting (100 req/min + 20 burst) using Redis
- Implement circuit breakers for DB and Redis to prevent cascading failures

This enables the service to scale horizontally with multiple replicas
behind a load balancer without exhausting database connections or
maintaining separate token caches per instance.

db/db.go

@@ -1,6 +1,7 @@
 package db
 
 import (
+	"authorization/helper"
 	"database/sql"
 	"fmt"
 	"log"
@@ -13,6 +14,9 @@ import (
 // DB is the global database connection pool
 var DB *sql.DB
 
+// DBCircuitBreaker protects database operations
+var DBCircuitBreaker *helper.CircuitBreaker
+
 func InitDB() (*sql.DB, error) {
 	// Get connection details from environment variables (loaded in main)
 	connStr := fmt.Sprintf("%s:%s@tcp(%s:%s)/%s?parseTime=true",
@@ -29,13 +33,20 @@ func InitDB() (*sql.DB, error) {
 	if err != nil {
 		return nil, fmt.Errorf("error opening database: %v", err)
 	}
-	// Set connection pool parameters
-	DB.SetMaxOpenConns(100)                // Maximum number of open connections to the database
-	DB.SetMaxIdleConns(100)                // Maximum number of connections in the idle connection pool
+	// Initialize circuit breaker
+	DBCircuitBreaker = helper.NewCircuitBreaker("database", 5, 2*time.Second)
+
+	// Set connection pool parameters optimized for horizontal scaling
+	// Lower per-replica to allow more replicas without exhausting DB connections
+	DB.SetMaxOpenConns(25)                 // Maximum number of open connections to the database
+	DB.SetMaxIdleConns(10)                 // Maximum number of connections in the idle connection pool
 	DB.SetConnMaxLifetime(5 * time.Minute) // Maximum amount of time a connection may be reused
 
-	// Check if the database connection is working
-	if err := DB.Ping(); err != nil {
+	// Check if the database connection is working with circuit breaker
+	err = DBCircuitBreaker.Call(func() error {
+		return DB.Ping()
+	})
+	if err != nil {
 		log.Printf("Database connection lost: %v. Reconnecting...", err)
 		DB, err = InitDB()
 		if err != nil {