feat(infra): create staging docker-compose and environment configuration (#67)

This pull request introduces a production-grade, security-hardened Docker Staging environment configuration for **NexusReader**, prepared directly from the `develop` branch.

### 🚀 Key Additions

1. **`docker-compose.stage.yml`**:
   - Deploys five isolated containers (`nexus-web-stage`, `nexus-db-stage`, `nexus-qdrant-stage`, `nexus-neo4j-stage`) inside a dedicated `nexus-stage` bridge network.
   - Sets non-conflicting port mappings to allow staging to run concurrently with other environments on the same host (e.g., Web on `5080`, Postgres on `5438`, Neo4j HTTP on `7488`).
   - Configures robust container healthchecks (`curl` for Qdrant, `wget` for Neo4j, `pg_isready` for Postgres).
   - Maps dedicated named persistent volumes for databases (`pgdata_stage`, `qdrant_stage_data`, `neo4j_stage_data`) to prevent data loss.
   - Maps separate persistent volumes specifically for dynamic web uploads (`stage_www_uploads` for EPUBs, `stage_www_covers` for covers) without overriding the compiled static web client files.

2. **`.env.stage.template`**:
   - A clean deployment environment template providing a blueprint of all variables.
   - Copied to `.env.stage` locally during deployment to inject secrets securely.
   - Mandates a secure `NEXUS_ADMIN_PASSWORD` (checked by `DbInitializer` for staging/production builds).

3. **`.gitignore`**:
   - Explicitly ignores local environment configurations (such as `.env.stage`) to prevent accidentally committing credentials, while keeping the `.env.stage.template` tracked.

---

### 🧪 Verification Performed

- **Docker Compose Validation**: Ran `docker compose -f docker-compose.stage.yml --env-file .env.stage config` successfully with zero configuration or parsing errors.
- **Solution Compilation**: Ran `dotnet build NexusReader.slnx --no-restore` from root — **SUCCESS** with `0` compile errors.
- **Automated Tests**: Ran `dotnet test --no-restore` — **SUCCESS** (all 20/20 unit tests passed).

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #67
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

.env.stage.template

@@ -0,0 +1,42 @@
+# ===================================================================
+# NexusReader — Staging (Stage) Environment Variables
+# ===================================================================
+# Copy this file to `.env.stage` and fill in the values before deployment:
+#   cp .env.stage.template .env.stage
+#
+# Then deploy with:
+#   docker compose -f docker-compose.stage.yml --env-file .env.stage up -d --build
+# ===================================================================
+
+# === PostgreSQL ===
+POSTGRES_USER=nexus_user_stage
+POSTGRES_PASSWORD=CHANGE_ME_TO_STRONG_PASSWORD
+POSTGRES_DB=nexus_stage_db
+POSTGRES_PORT=5438
+
+# === Neo4j ===
+NEO4J_USERNAME=neo4j
+NEO4J_PASSWORD=CHANGE_ME_TO_STRONG_PASSWORD
+
+# === Qdrant (leave empty to disable API key auth in staging) ===
+QDRANT_API_KEY=
+
+# === Web App ===
+WEB_PORT=5080
+
+# === Google OAuth (Staging credentials) ===
+GOOGLE_CLIENT_ID=placeholder_google_client_id_stage
+GOOGLE_CLIENT_SECRET=placeholder_google_client_secret_stage
+
+# === Gemini AI ===
+GOOGLE_AI_API_KEY=placeholder_gemini_api_key_stage
+
+# === Secure Admin Seed Password (MANDATORY in Staging environment) ===
+# This password is used by DbInitializer during startup. Cannot be empty or 'Admin123!'.
+NEXUS_ADMIN_PASSWORD=CHANGE_ME_TO_SECURE_ADMIN_PASSWORD
+
+# === Non-standard ports for auxiliary services ===
+QDRANT_HTTP_PORT=6383
+QDRANT_GRPC_PORT=6384
+NEO4J_HTTP_PORT=7488
+NEO4J_BOLT_PORT=7688