docs: add project README

README.md

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+# garge-api
+
+ASP.NET Core 8 REST API for the Garge garage monitoring system. Handles authentication, sensor data persistence, automation rule evaluation, and orchestrates switch commands to garge-operator via webhooks.
+
+## What it does
+
+- Stores sensor readings (battery voltage, temperature, humidity) in PostgreSQL
+- Evaluates automation rules and triggers switch commands (e.g. start charger when battery drops below 12.5 V)
+- Fetches hourly electricity prices from Nord Pool for demand-responsive automations
+- Sends webhooks to garge-operator when a switch should toggle
+- Manages user accounts and authentication (JWT + refresh tokens)
+
+## Tech stack
+
+- **ASP.NET Core 8** Web API
+- **PostgreSQL** — Entity Framework Core 9 (Npgsql)
+- **ASP.NET Identity** + JWT Bearer tokens
+- **AutoMapper** — entity ↔ DTO mapping
+- **Serilog** — structured logging
+- **Swagger** — API docs at `/swagger`
+
+## Configuration
+
+Secrets are set via environment variables (or .NET User Secrets in development):
+
+| Key | Description |
+|-----|-------------|
+| `ConnectionStrings__DefaultConnection` | PostgreSQL connection string |
+| `Jwt__Key` | Secret key for signing JWT tokens |
+| `Jwt__Issuer` / `Jwt__Audience` | JWT issuer and audience |
+| `Email__ApiKey` | SendGrid or Brevo API key |
+
+## Running
+
+```bash
+dotnet ef database update   # apply migrations
+dotnet run
+```
+
+API docs available at `http://localhost:5277/swagger`.