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>

.env.test.template

@@ -0,0 +1,41 @@
+# ===================================================================
+# NexusReader — Test Environment Variables
+# ===================================================================
+# Copy this file to `.env` and fill in the values before deployment:
+#   cp .env.test.template .env
+#
+# Then deploy with:
+#   docker compose -f docker-compose.test.yml up -d --build
+# ===================================================================
+
+# === PostgreSQL ===
+POSTGRES_USER=nexus_user
+POSTGRES_PASSWORD=CHANGE_ME_TO_STRONG_PASSWORD
+POSTGRES_DB=nexus_test_db
+POSTGRES_PORT=5433
+
+# === Neo4j ===
+NEO4J_USERNAME=neo4j
+NEO4J_PASSWORD=CHANGE_ME_TO_STRONG_PASSWORD
+
+# === Qdrant (leave empty to disable API key auth) ===
+QDRANT_API_KEY=
+
+# === Web App ===
+WEB_PORT=5050
+
+# === Google OAuth (placeholder for test) ===
+GOOGLE_CLIENT_ID=placeholder
+GOOGLE_CLIENT_SECRET=placeholder
+
+# === Gemini AI (placeholder for test) ===
+GOOGLE_AI_API_KEY=placeholder
+
+# === Admin Seed Password ===
+NEXUS_ADMIN_PASSWORD=CHANGE_ME
+
+# === Non-standard ports for auxiliary services ===
+QDRANT_HTTP_PORT=6343
+QDRANT_GRPC_PORT=6344
+NEO4J_HTTP_PORT=7484
+NEO4J_BOLT_PORT=7697