feat(rate-limiting): Implement core rate limiting functionality with configuration, decision-making, metrics, middleware, and service registration

- Add RateLimitConfig for configuration management with YAML binding support.
- Introduce RateLimitDecision to encapsulate the result of rate limit checks.
- Implement RateLimitMetrics for OpenTelemetry metrics tracking.
- Create RateLimitMiddleware for enforcing rate limits on incoming requests.
- Develop RateLimitService to orchestrate instance and environment rate limit checks.
- Add RateLimitServiceCollectionExtensions for dependency injection registration.

docs/modules/vexlens/operations/deployment.md

@@ -11,7 +11,7 @@
 | Component | Requirement | Notes |
 |-----------|-------------|-------|
 | Runtime | .NET 10.0+ | LTS recommended |
-| Database | MongoDB 6.0+ | For projections and issuer directory |
+| Database | PostgreSQL 15.0+ | For projections and issuer directory |
 | Cache | Redis 7.0+ (optional) | For caching consensus results |
 | Memory | 512MB minimum | 2GB recommended for production |
 | CPU | 2 cores minimum | 4 cores for high throughput |
@@ -43,13 +43,12 @@ VEXLENS_TRUST_ALLOW_UNKNOWN_ISSUERS=true
 VEXLENS_TRUST_UNKNOWN_ISSUER_PENALTY=0.5
 
 # Storage
-VEXLENS_STORAGE_MONGODB_CONNECTION_STRING=mongodb://localhost:27017
-VEXLENS_STORAGE_MONGODB_DATABASE=vexlens
+VEXLENS_STORAGE_POSTGRESQL_CONNECTION_STRING=Host=localhost;Database=vexlens;Username=stellaops;Password=secret
 VEXLENS_STORAGE_PROJECTION_RETENTION_DAYS=365
 VEXLENS_STORAGE_EVENT_RETENTION_DAYS=90
 
 # Issuer Directory
-VEXLENS_ISSUER_DIRECTORY_SOURCE=mongodb
+VEXLENS_ISSUER_DIRECTORY_SOURCE=postgresql
 VEXLENS_ISSUER_DIRECTORY_REFRESH_INTERVAL_MINUTES=60
 
 # Observability
@@ -86,16 +85,15 @@ vexlens:
       ProductAuthority: 0.05
 
   storage:
-    mongodb:
-      connectionString: mongodb://localhost:27017
-      database: vexlens
-      projectionsCollection: consensus_projections
-      issuersCollection: issuers
+    postgresql:
+      connectionString: Host=localhost;Database=vexlens;Username=stellaops;Password=secret
+      projectionsTable: consensus_projections
+      issuersTable: issuers
     projectionRetentionDays: 365
     eventRetentionDays: 90
 
   issuerDirectory:
-    source: mongodb
+    source: postgresql
     refreshIntervalMinutes: 60
     seedFile: /etc/vexlens/issuers.json
 
@@ -126,7 +124,7 @@ docker run -d \
   --name vexlens \
   -p 8080:8080 \
   -v /etc/vexlens:/etc/vexlens:ro \
-  -e VEXLENS_STORAGE_MONGODB_CONNECTION_STRING=mongodb://mongo:27017 \
+  -e VEXLENS_STORAGE_POSTGRESQL_CONNECTION_STRING="Host=postgres;Database=vexlens;Username=stellaops;Password=secret" \
   stellaops/vexlens:latest
 ```
 
@@ -154,11 +152,11 @@ spec:
         ports:
         - containerPort: 8080
         env:
-        - name: VEXLENS_STORAGE_MONGODB_CONNECTION_STRING
+        - name: VEXLENS_STORAGE_POSTGRESQL_CONNECTION_STRING
           valueFrom:
             secretKeyRef:
               name: vexlens-secrets
-              key: mongodb-connection-string
+              key: postgresql-connection-string
         resources:
           requests:
             memory: "512Mi"
@@ -205,7 +203,7 @@ spec:
 ```bash
 helm install vexlens stellaops/vexlens \
   --namespace stellaops \
-  --set mongodb.connectionString=mongodb://mongo:27017 \
+  --set postgresql.connectionString="Host=postgres;Database=vexlens;Username=stellaops;Password=secret" \
   --set replicas=2 \
   --set resources.requests.memory=512Mi \
   --set resources.limits.memory=2Gi
@@ -293,7 +291,7 @@ curl http://vexlens:8080/health/live
 
 ```bash
 curl http://vexlens:8080/health/ready
-# Response: {"status": "Healthy", "checks": {"mongodb": "Healthy", "issuerDirectory": "Healthy"}}
+# Response: {"status": "Healthy", "checks": {"postgresql": "Healthy", "issuerDirectory": "Healthy"}}
 ```
 
 ### 5.3 Detailed Health
@@ -358,11 +356,10 @@ groups:
 ### 7.1 Backup Projections
 
 ```bash
-# MongoDB backup
-mongodump --uri="mongodb://localhost:27017" \
-  --db=vexlens \
-  --collection=consensus_projections \
-  --out=/backup/vexlens-$(date +%Y%m%d)
+# PostgreSQL backup
+pg_dump -h localhost -U stellaops -d vexlens \
+  -t consensus_projections \
+  -F c -f /backup/vexlens-projections-$(date +%Y%m%d).dump
 ```
 
 ### 7.2 Backup Issuer Directory
@@ -376,10 +373,9 @@ curl http://vexlens:8080/api/v1/vexlens/issuers?limit=1000 \
 ### 7.3 Restore
 
 ```bash
-# Restore MongoDB
-mongorestore --uri="mongodb://localhost:27017" \
-  --db=vexlens \
-  /backup/vexlens-20251206/
+# Restore PostgreSQL
+pg_restore -h localhost -U stellaops -d vexlens \
+  /backup/vexlens-projections-20251206.dump
 
 # Re-seed issuers if needed
 # Issuers are automatically loaded from seed file on startup
@@ -408,10 +404,10 @@ vexlens:
     batchTimeoutMs: 50
 
   storage:
-    mongodb:
+    postgresql:
       # Connection pool
-      maxConnectionPoolSize: 100
-      minConnectionPoolSize: 10
+      maxPoolSize: 100
+      minPoolSize: 10
 
   caching:
     enabled: true