docs: update README for Gradle at root, add convenience task docs

Update all references to match the new repo-root Gradle layout
after moving the wrapper out of backend/.

- Quick Start: add ./gradlew up alternative and hint at ./gradlew check
- Spring profiles: ./gradlew bootRun → ./gradlew :backend:bootRun
- Development section: add All-in-one subsection with check/up/down/reset
- Backend dev: cd backend && ./gradlew bootRun → ./gradlew :backend:bootRun
- Development vs Production table: ./gradlew bootRun → ./gradlew :backend:bootRun
- Project Structure tree: add gradlew, gradle/, settings.gradle, build.gradle
- Remove ARCHITECTURE.md reference (file never existed)
- Add Database reset section with ./gradlew reset

Also add .gradle/ and build/ to .gitignore with gradle-wrapper.jar
exception (was staged but not committed with previous refactor).

.gitignore

@@ -10,6 +10,7 @@ target/
 *.jar
 *.war
 !.mvn/wrapper/maven-wrapper.jar
+!gradle/wrapper/gradle-wrapper.jar
 
 # Environment
 .env

README.md

@@ -34,7 +34,7 @@ The user enters a registration number, composes a letter (from a template or fre
 git clone <repo-url> bilhej
 cd bilhej
 cp .env.example .env          # fill in your keys
-docker compose up -d
+docker compose up -d          # or: ./gradlew up
 ```
 
 The app will be available at:
@@ -72,7 +72,7 @@ No CORS configuration needed in development — the browser never calls the back
 **Spring profiles:**
 | Profile | Datasource | Use |
 |---------|-----------|-----|
-| default | H2 in-memory | Local IDE dev (`./gradlew bootRun`) |
+| default | H2 in-memory | Local IDE dev (`./gradlew :backend:bootRun`) |
 | `docker` | PostgreSQL via `docker-compose.yml` | Docker Compose dev |
 | `prod` | PostgreSQL (production config) | Deploy (`docker-compose.prod.yml`) |
 
@@ -130,17 +130,21 @@ bilhej/
 ├── docker-compose.yml         # dev: postgres + backend (bootRun) + frontend (Vite HMR)
 ├── docker-compose.prod.yml    # prod: multi-stage builds, no source mounts, restart: unless-stopped
 ├── docker/
-│   ├── backend.Dockerfile         # dev: JDK + gradle bootRun
+│   ├── backend.Dockerfile         # dev: JDK + gradle :backend:bootRun
 │   ├── backend.prod.Dockerfile    # prod: multi-stage (Gradle → JRE Alpine, non-root)
 │   ├── frontend.Dockerfile        # dev: Node + vite dev server
 │   ├── frontend.prod.Dockerfile   # prod: multi-stage (Node → nginx)
 │   ├── nginx.conf                 # prod: SPA fallback + /api proxy
 │   └── entrypoint.sh              # prod: self-signed cert generation
+├── gradlew                    # Gradle wrapper (run from repo root)
+├── gradle/
+│   └── wrapper/
+├── settings.gradle            # rootProject.name + include 'backend'
+├── build.gradle               # convenience tasks: check, up, down, reset
 ├── .env.example
 ├── README.md
 ├── REQUIREMENTS.md
-├── CODING_GUIDELINES.md
-└── ARCHITECTURE.md
+└── CODING_GUIDELINES.md
 ```
 
 ---
@@ -149,7 +153,7 @@ bilhej/
 
 | Aspect | `docker compose up -d` | `docker compose -f docker-compose.prod.yml up -d` |
 |--------|------------------------|---------------------------------------------------|
-| Backend | `./gradlew bootRun` (compiles on change) | Multi-stage build → `java -jar app.jar` |
+| Backend | `./gradlew :backend:bootRun` (compiles on change) | Multi-stage build → `java -jar app.jar` |
 | Backend image | `eclipse-temurin:21-jdk` (~400 MB) | `eclipse-temurin:21-jre-alpine` (~200 MB) |
 | Backend user | root | `bilhej` (non-root) |
 | Frontend | Vite dev server (HMR, `--host 0.0.0.0`) | nginx serving static `dist/` |
@@ -163,19 +167,27 @@ bilhej/
 
 ## Development
 
+### All-in-one (from repo root)
+
+```bash
+./gradlew check         # lint → frontend test → backend test → integration test
+./gradlew up            # docker compose up -d
+./gradlew down          # docker compose down
+./gradlew reset         # docker compose down -v && docker compose up -d (full DB reset)
+```
+
 ### Frontend (dev server with HMR)
 
 ```bash
 cd frontend
-npm install
-npm run dev
+npm install          # first time only
+npm run dev          # :3000 with HMR
 ```
 
 ### Backend (IDE or CLI)
 
 ```bash
-cd backend
-./gradlew bootRun
+./gradlew :backend:bootRun    # :8080, profile: default (H2)
 ```
 
 ### Stripe Webhooks (local testing)
@@ -184,10 +196,15 @@ cd backend
 stripe listen --forward-to localhost:8080/api/webhooks/stripe
 ```
 
+### Database reset
+
+```bash
+./gradlew reset    # wipes DB volume and restarts containers
+```
+
 ---
 
 ## Related Documents
 
 - [REQUIREMENTS.md](./REQUIREMENTS.md) — Full product requirements and business model
 - [CODING_GUIDELINES.md](./CODING_GUIDELINES.md) — Code conventions and standards
-- [ARCHITECTURE.md](./ARCHITECTURE.md) — Detailed architecture and data flow