📝 docs: mkcert local HTTPS setup + Makefile cert target (ADR-0028 Phase B prep)

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+# mkcert: Local HTTPS for Development
+
+## Overview
+
+This document describes how to set up local HTTPS development certificates using `mkcert`.
+
+OIDC providers **reject `http://localhost` as a redirect URI** by default for security reasons. To test OAuth 2.0 / OpenID Connect flows locally, the development server must be accessible via HTTPS. `mkcert` provides a zero-configuration local Certificate Authority that generates trusted certificates for localhost and custom domains.
+
+This setup is a prerequisite for **ADR-0028 Phase B** (OpenID Connect Authorization Code flow).
+
+## Why mkcert
+
+- **Trusted locally**: Certificates are automatically trusted by the system root store (macOS, Linux, Windows)
+- **No configuration**: Single commands to create and install the CA
+- **Local-only**: Certificates are valid only for localhost development, never exposed to production
+- **Industry standard**: Widely adopted tool for local HTTPS development
+
+## Installation
+
+### macOS (Homebrew)
+
+```bash
+brew install mkcert
+mkcert -install
+```
+
+The `mkcert -install` command creates and installs a local Certificate Authority in your system trust store.
+
+### Linux
+
+See [mkcert GitHub](https://github.com/FiloSottile/mkcert#installation) for distribution-specific instructions.
+
+### Windows
+
+See [mkcert GitHub](https://github.com/FiloSottile/mkcert#installation) for Windows installation.
+
+## Generate Certificates
+
+Use the provided Make target to generate certificates for localhost development:
+
+```bash
+make cert
+```
+
+This runs the following command:
+
+```bash
+mkcert -cert-file ./certs/dev-cert.pem -key-file ./certs/dev-key.pem localhost 127.0.0.1 ::1
+```
+
+### Output Files
+
+| File | Description |
+|------|-------------|
+| `./certs/dev-cert.pem` | TLS certificate for localhost, 127.0.0.1, and ::1 |
+| `./certs/dev-key.pem` | Private key for the certificate |
+
+Both files are created in the `./certs/` directory at the project root.
+
+## Use in Development
+
+Once certificates are generated, start the server with TLS enabled:
+
+```bash
+./bin/server --tls-cert ./certs/dev-cert.pem --tls-key ./certs/dev-key.pem
+```
+
+> **Note**: The `--tls-cert` and `--tls-key` flags are **not yet implemented** — this is planned for ADR-0028 Phase B.4. The Makefile and certificate generation are prepared in advance so that when the server TLS support is added, the certificates are ready.
+
+The server will then be accessible at:
+- `https://localhost:8080` (or the configured port)
+- `https://127.0.0.1:8080`
+- `https://[::1]:8080`
+
+All OIDC callback URLs must use HTTPS with one of these hostnames.
+
+## Clean Up
+
+To remove generated certificates:
+
+```bash
+make clean-cert
+```
+
+This deletes the entire `./certs/` directory.
+
+## .gitignore
+
+The `certs/` directory contains locally-generated certificates and **must not be committed** to version control.
+
+Ensure `certs/` is in your `.gitignore`. If it is not already present, add it:
+
+```bash
+echo "certs/" >> .gitignore
+```
+
+## Cross-References
+
+- [ADR-0028: Passwordless authentication: magic link → OpenID Connect](../adr/0028-passwordless-auth-migration.md) — Phase B describes the OIDC implementation that requires HTTPS
+- [mkcert GitHub Repository](https://github.com/FiloSottile/mkcert) — Official documentation
+
+## Troubleshooting
+
+### "mkcert not found" when running `make cert`
+
+Ensure `mkcert` is installed and available in your `PATH`. The Makefile checks for this and will display an error message if `mkcert` is not found.
+
+### Certificate not trusted by browser
+
+Run `mkcert -install` again. On macOS, you may need to restart your browser completely (close all windows, not just tabs).
+
+### Port already in use
+
+If another process is using the port (e.g., a non-TLS server on port 8080), stop that process first or configure the server to use a different port.
+
+## See Also
+
+- `make help` — List all available Make targets
+- [documentation/API.md](API.md) — API endpoints reference
+- [documentation/BDD_GUIDE.md](BDD_GUIDE.md) — BDD testing guide