Implement PID-controlled adaptive rate limiting for database operations

- Add LoadMonitor interface in pkg/interfaces/loadmonitor/ for database load metrics
- Implement PIDController with filtered derivative to suppress high-frequency noise
  - Proportional (P): immediate response to current error
  - Integral (I): eliminates steady-state offset with anti-windup clamping
  - Derivative (D): rate-of-change prediction with low-pass filtering
- Create BadgerLoadMonitor tracking L0 tables, compaction score, and cache hit ratio
- Create Neo4jLoadMonitor tracking query semaphore usage and latencies
- Add AdaptiveRateLimiter combining PID controllers for reads and writes
- Configure via environment variables:
  - ORLY_RATE_LIMIT_ENABLED: enable/disable rate limiting
  - ORLY_RATE_LIMIT_TARGET_MB: target memory limit (default 1500MB)
  - ORLY_RATE_LIMIT_*_K[PID]: PID gains for reads/writes
  - ORLY_RATE_LIMIT_MAX_*_MS: maximum delays
  - ORLY_RATE_LIMIT_*_TARGET: setpoints for reads/writes
- Integrate rate limiter into Server struct and lifecycle management
- Add comprehensive unit tests for PID controller behavior

Files modified:
- app/config/config.go: Add rate limiting configuration options
- app/main.go: Initialize and start/stop rate limiter
- app/server.go: Add rateLimiter field to Server struct
- main.go: Create rate limiter with appropriate monitor
- pkg/run/run.go: Pass disabled limiter for test instances
- pkg/interfaces/loadmonitor/: New LoadMonitor interface
- pkg/ratelimit/: New PID controller and limiter implementation

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

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

pkg/run/run.go

@@ -16,6 +16,7 @@ import (
 	"next.orly.dev/app/config"
 	"next.orly.dev/pkg/acl"
 	"next.orly.dev/pkg/database"
+	"next.orly.dev/pkg/ratelimit"
 )
 
 // Options configures relay startup behavior.
@@ -126,8 +127,11 @@ func Start(cfg *config.C, opts *Options) (relay *Relay, err error) {
 	}
 	acl.Registry.Syncer()
 
+	// Create rate limiter (disabled for test relay instances)
+	limiter := ratelimit.NewDisabledLimiter()
+
 	// Start the relay
-	relay.quit = app.Run(relay.ctx, cfg, relay.db)
+	relay.quit = app.Run(relay.ctx, cfg, relay.db, limiter)
 
 	return
 }