feat(infra): Docker-compose configuration and environment-specific security guards for Beta deployment to Test environment (#56)

This pull request introduces the dedicated containerized infrastructure and configuration for deploying NexusReader's beta version in the Test environment.

### Summary of Changes

1. **Docker Infrastructure & Secrets**:
   - **`docker-compose.test.yml`**: Configured dedicated database and auxiliary services (PostgreSQL 17, Qdrant, Neo4j) on isolated, non-standard ports to ensure zero conflict with the existing server configurations.
   - **`.env.test.template`**: Provided an environment variable template showing required setups, including mandatory database passwords, API keys, and admin custom passwords.
   - **`.gitignore`**: Excluded local `.env` files to prevent accidental commits of production or staging secrets.

2. **Database Hardening**:
   - Configured Neo4j with basic authentication (`IDriver` instantiation uses basic auth when credentials are provided in configuration).
   - Configured PostgreSQL to use mandatory authentication.
   - Configured the admin seeder (`DbInitializer.cs`) to dynamically use `NEXUS_ADMIN_PASSWORD` from environment variables, falling back to a default password in local Development only.

3. **Feature-Flagged Restrictions**:
   - **`appsettings.Test.json`**: Implemented `Features:AllowRegistration` and `Features:AllowPasswordReset` flags set to `false`.
   - **Middleware Enforcement (`Program.cs`)**: Intercepts requests to `/identity/register` and `/identity/forgotPassword` (and their MVC/form variations) and rejects them with a `403 Forbidden` response in restricted environments.
   - **OAuth Provisioning Guard (`Program.cs`)**: Blocks new account provisioning via Google OAuth callback by checking the `Features:AllowRegistration` configuration, redirecting users to the login page with a descriptive error.
   - **UI Protection (`Login.razor`, `Register.razor`)**: Conditionally hides registration/password reset links and intercepts manual navigation attempts to `/account/register` by redirecting to login with a warning.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #56
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

docker-compose.test.yml

@@ -0,0 +1,97 @@
+services:
+  db:
+    image: pgvector/pgvector:pg17
+    container_name: nexus-db-test
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER:-nexus_user}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?POSTGRES_PASSWORD is required}
+      POSTGRES_DB: ${POSTGRES_DB:-nexus_test_db}
+    ports:
+      - "${POSTGRES_PORT:-5433}:5432"
+    volumes:
+      - pgdata_test:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-nexus_user} -d ${POSTGRES_DB:-nexus_test_db}"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+    networks:
+      - nexus-test
+    restart: unless-stopped
+
+  web:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    container_name: nexus-web-test
+    ports:
+      - "${WEB_PORT:-5050}:5000"
+    environment:
+      - ASPNETCORE_ENVIRONMENT=Test
+      - ConnectionStrings__PostgresConnection=Host=db;Database=${POSTGRES_DB:-nexus_test_db};Username=${POSTGRES_USER:-nexus_user};Password=${POSTGRES_PASSWORD:?POSTGRES_PASSWORD is required}
+      - ConnectionStrings__QdrantConnection=http://qdrant:6334
+      - ConnectionStrings__Neo4jConnection=bolt://neo4j:7687
+      - Neo4j__Username=${NEO4J_USERNAME:-neo4j}
+      - Neo4j__Password=${NEO4J_PASSWORD:?NEO4J_PASSWORD is required}
+      - Authentication__Google__ClientId=${GOOGLE_CLIENT_ID:-placeholder}
+      - Authentication__Google__ClientSecret=${GOOGLE_CLIENT_SECRET:-placeholder}
+      - Ai__Google__ApiKey=${GOOGLE_AI_API_KEY:-placeholder}
+      - NEXUS_ADMIN_PASSWORD=${NEXUS_ADMIN_PASSWORD:?NEXUS_ADMIN_PASSWORD is required}
+    depends_on:
+      db:
+        condition: service_healthy
+      qdrant:
+        condition: service_healthy
+      neo4j:
+        condition: service_healthy
+    networks:
+      - nexus-test
+    restart: unless-stopped
+
+  qdrant:
+    image: qdrant/qdrant:latest
+    container_name: nexus-qdrant-test
+    environment:
+      - QDRANT__SERVICE__API_KEY=${QDRANT_API_KEY:-}
+    ports:
+      - "${QDRANT_HTTP_PORT:-6343}:6333"
+      - "${QDRANT_GRPC_PORT:-6344}:6334"
+    volumes:
+      - qdrant_test_data:/qdrant/storage
+    healthcheck:
+      test: ["CMD-SHELL", "curl -sf http://localhost:6333/healthz || exit 1"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+    networks:
+      - nexus-test
+    restart: unless-stopped
+
+  neo4j:
+    image: neo4j:5-community
+    container_name: nexus-neo4j-test
+    environment:
+      - NEO4J_AUTH=${NEO4J_USERNAME:-neo4j}/${NEO4J_PASSWORD:?NEO4J_PASSWORD is required}
+    ports:
+      - "${NEO4J_HTTP_PORT:-7484}:7474"
+      - "${NEO4J_BOLT_PORT:-7697}:7687"
+    volumes:
+      - neo4j_test_data:/data
+    healthcheck:
+      test: ["CMD-SHELL", "wget -qO- http://localhost:7474 || exit 1"]
+      interval: 10s
+      timeout: 10s
+      retries: 10
+      start_period: 30s
+    networks:
+      - nexus-test
+    restart: unless-stopped
+
+volumes:
+  pgdata_test:
+  qdrant_test_data:
+  neo4j_test_data:
+
+networks:
+  nexus-test:
+    driver: bridge