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>

app/main.go

@@ -21,12 +21,13 @@ import (
 	"next.orly.dev/pkg/protocol/graph"
 	"next.orly.dev/pkg/protocol/nip43"
 	"next.orly.dev/pkg/protocol/publish"
+	"next.orly.dev/pkg/ratelimit"
 	"next.orly.dev/pkg/spider"
 	dsync "next.orly.dev/pkg/sync"
 )
 
 func Run(
-	ctx context.Context, cfg *config.C, db database.Database,
+	ctx context.Context, cfg *config.C, db database.Database, limiter *ratelimit.Limiter,
 ) (quit chan struct{}) {
 	quit = make(chan struct{})
 	var once sync.Once
@@ -64,14 +65,15 @@ func Run(
 	}
 	// start listener
 	l := &Server{
-		Ctx:        ctx,
-		Config:     cfg,
-		DB:         db,
-		publishers: publish.New(NewPublisher(ctx)),
-		Admins:     adminKeys,
-		Owners:     ownerKeys,
-		cfg:        cfg,
-		db:         db,
+		Ctx:         ctx,
+		Config:      cfg,
+		DB:          db,
+		publishers:  publish.New(NewPublisher(ctx)),
+		Admins:      adminKeys,
+		Owners:      ownerKeys,
+		rateLimiter: limiter,
+		cfg:         cfg,
+		db:          db,
 	}
 
 	// Initialize NIP-43 invite manager if enabled
@@ -360,6 +362,12 @@ func Run(
 		}
 	}
 
+	// Start rate limiter if enabled
+	if limiter != nil && limiter.IsEnabled() {
+		limiter.Start()
+		log.I.F("adaptive rate limiter started")
+	}
+
 	// Wait for database to be ready before accepting requests
 	log.I.F("waiting for database warmup to complete...")
 	<-db.Ready()
@@ -457,6 +465,12 @@ func Run(
 			log.I.F("directory spider stopped")
 		}
 
+		// Stop rate limiter if running
+		if l.rateLimiter != nil && l.rateLimiter.IsEnabled() {
+			l.rateLimiter.Stop()
+			log.I.F("rate limiter stopped")
+		}
+
 		// Create shutdown context with timeout
 		shutdownCtx, cancelShutdown := context.WithTimeout(context.Background(), 10*time.Second)
 		defer cancelShutdown()