Architecture: cost optimisation, blue-green deployment, reduce to 3 projects

- Reduce from 4 to 3 GCP projects (drop sandbox, use staging namespaces)
- Add blue-green deployment strategy via Argo Rollouts
- Add cost optimisation section with monthly estimate (~$175-245)
- Add blue-green flow diagram and cost pie chart to HLD

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/architecture-hld.md

@@ -9,22 +9,25 @@ flowchart TB
     end
 
     subgraph GCP["Google Cloud Platform"]
-        subgraph Projects["Project Structure"]
+        subgraph Projects["Project Structure (3 projects)"]
             Prod[company-inc-prod]
-            Staging[company-inc-staging]
+            Staging[company-inc-staging<br/>QA + dev namespaces]
             Shared[company-inc-shared]
-            Sandbox[company-inc-sandbox]
         end
 
         subgraph Edge["Edge / Networking"]
             LB[Cloud Load Balancer<br/>HTTPS · TLS termination]
             CDN[Cloud CDN<br/>Static Assets]
-            NAT[Cloud NAT<br/>Egress]
+            NAT[Cloud NAT<br/>Egress · shared]
         end
 
         subgraph VPC["VPC — Private Subnets"]
             subgraph GKE["GKE Autopilot Cluster"]
                 Ingress[Ingress Controller]
+                subgraph BlueGreen["Blue-Green Deployment"]
+                    Green[Green — stable<br/>receives traffic]
+                    Blue[Blue — new release<br/>smoke tests]
+                end
                 subgraph Workloads
                     API[Backend — Python / Flask<br/>HPA · 2–3 replicas]
                     SPA[Frontend — React SPA<br/>Nginx]
@@ -44,14 +47,17 @@ flowchart TB
     subgraph CICD["CI / CD"]
         Git[Git Repository]
         Actions[Gitea / GitHub Actions<br/>Build · Test · Scan]
-        Argo[ArgoCD / Flux<br/>GitOps Deploy]
+        Argo[ArgoCD + Argo Rollouts<br/>GitOps · Blue-Green]
     end
 
     Users --> LB
     Users --> CDN
     LB --> Ingress
     CDN --> SPA
-    Ingress --> API
+    Ingress -->|traffic| Green
+    Ingress -.->|after switch| Blue
+    Green --> API
+    Blue --> API
     Ingress --> SPA
     API --> Redis
     API --> Mongo
@@ -64,6 +70,24 @@ flowchart TB
     Argo --> GKE
 ```
 
+## Blue-Green Deployment Flow
+
+```mermaid
+flowchart LR
+    subgraph Cluster["GKE Cluster"]
+        LB[Load Balancer<br/>Service Selector]
+        Green[Green — v1.2.0<br/>current stable]
+        Blue[Blue — v1.3.0<br/>new release]
+    end
+
+    Deploy[ArgoCD<br/>Argo Rollouts] -->|deploy new version| Blue
+    Blue -->|smoke tests| Check{Tests pass?}
+    Check -->|yes| LB
+    LB -->|switch 100%| Blue
+    Check -->|no| Rollback[Rollback<br/>keep Green]
+    LB -.->|instant rollback| Green
+```
+
 ## CI / CD Pipeline
 
 ```mermaid
@@ -72,8 +96,8 @@ flowchart LR
     Repo -->|webhook| CI[CI Pipeline<br/>lint · test · build]
     CI -->|push image| Registry[Artifact Registry]
     CI -->|update manifests| GitOps[GitOps Repo]
-    GitOps -->|sync| Argo[ArgoCD / Flux]
-    Argo -->|deploy| GKE[GKE Cluster]
+    GitOps -->|sync| Argo[ArgoCD]
+    Argo -->|blue-green deploy| GKE[GKE Cluster]
 ```
 
 ## Network Security Layers
@@ -86,3 +110,13 @@ flowchart TD
     NP --> Pods[Application Pods<br/>Private IPs only]
     Pods --> PE[Private Endpoint<br/>MongoDB Atlas]
 ```
+
+## Cost Profile (Early Stage)
+
+```mermaid
+pie title Monthly Cost Breakdown (~$200)
+    "GKE Autopilot" : 120
+    "MongoDB Atlas M10" : 60
+    "LB + NAT" : 30
+    "Registry + Secrets" : 5
+```