docs: add CONTRIBUTING.md + local dev guide

CONTRIBUTING.md

@@ -0,0 +1,162 @@
+# Contributing to trood-leader
+
+Thanks for helping out. This guide gets you from a fresh checkout to a running
+local API with a working database and embedding server in a few minutes.
+
+## Prerequisites
+
+- **Node.js 20+** (the Dockerfile targets `node:20`)
+- **Docker + Docker Compose** (for the Postgres + API containers)
+- **npm** (ships with Node)
+
+## 1. Clone and install dependencies
+
+```bash
+git clone https://github.com/TroodInc/trood-leader
+cd trood-leader
+npm install
+```
+
+`npm install` runs in the repo root. The embedding server is a separate npm
+package under `embedding-server/` — install its dependencies too if you plan
+to run it locally (see step 4):
+
+```bash
+(cd embedding-server && npm install)
+```
+
+## 2. Configure environment
+
+Copy the template and fill the four service values:
+
+```bash
+cp .env.example .env
+```
+
+`DATABASE_URL` in `.env.example` points at the Postgres service declared in
+`docker-compose.yml`, which is correct inside the Docker Compose network. For
+host-side commands such as `npm run dev`, `npm run prisma:migrate`, or
+`npm run prisma:push`, use `localhost` instead:
+
+```bash
+DATABASE_URL=postgresql://trood:trood@localhost:5432/trood_leader
+```
+
+Values you must set before `/import` or `/export` endpoints work:
+
+| Variable                | Purpose                                                  |
+|-------------------------|----------------------------------------------------------|
+| `EMBEDDING_API_URL`     | URL of your embedding service (e.g. `http://host.docker.internal:3001` when the API runs in Docker) |
+| `EMBEDDING_VERSION`     | Version tag stored alongside each embedding              |
+| `EMAILOCTOPUS_API_KEY`  | API key for EmailOctopus exports                         |
+| `EMAILOCTOPUS_LIST_ID`  | Target list ID for EmailOctopus exports                  |
+
+## 3. Run the database and API
+
+The easiest path is Docker Compose, which brings up Postgres and the API
+together:
+
+```bash
+docker compose up --build
+```
+
+The API listens on `http://localhost:3000`. Postgres listens on
+`localhost:5432` with user/password/database `trood/trood/trood_leader`
+(matches `docker-compose.yml`).
+
+Docker Compose does not apply Prisma migrations automatically. On a fresh
+database, run step 4 once before using API endpoints.
+
+If you want to run the API outside Docker while keeping Postgres in Docker:
+
+```bash
+docker compose up -d postgres
+npm run dev          # ts-node-dev, reloads on save
+```
+
+Before running `npm run dev` on your host, make sure `DATABASE_URL` uses
+`localhost` as shown in step 2, not the Docker-only `postgres` hostname.
+
+## 4. Run migrations
+
+Prisma generates the client and applies migrations. These commands run on your
+host, so make sure `DATABASE_URL` uses `localhost` as shown in step 2:
+
+```bash
+npm run prisma:generate   # prisma generate
+npm run prisma:migrate    # prisma migrate dev (creates tables)
+```
+
+For quick iteration against an empty dev database you can use
+`npm run prisma:push` instead of `prisma:migrate` — it syncs the schema
+without creating a migration file.
+
+## 5. Start the embedding server (local dev)
+
+`embedding-server/` is a small Express service that speaks the `/embed`
+contract the API expects. It's useful for local dev so you don't need to
+point at a real embedding provider:
+
+```bash
+cd embedding-server
+npm start
+```
+
+It listens on `http://localhost:3001`. If the API runs on your host, set
+`EMBEDDING_API_URL` in `.env` to that URL. If the API runs in Docker Compose,
+use `http://host.docker.internal:3001` so the container can reach the host
+service.
+
+The default implementation returns a deterministic 384-dimension mock vector
+per text input, which is fine for end-to-end tests but obviously not a real
+embedding.
+
+## 6. Build for production
+
+```bash
+npm run build   # tsc -> dist/
+npm start       # node dist/index.js
+```
+
+## Troubleshooting
+
+**`ECONNREFUSED` on the embedding URL.** The API expects
+`EMBEDDING_API_URL` to point at a reachable embedding service. In Docker,
+`localhost` from inside the API container is the container itself, not your
+Mac — use `host.docker.internal:3001` or add the embedding server to
+`docker-compose.yml` so they share the network.
+
+**Prisma: `database "trood_leader" does not exist`.** The Postgres container
+creates the database from `POSTGRES_DB` on first start. If you ran Postgres
+without it, either `docker compose down -v` to reset the volume or
+`createdb -U trood trood_leader` manually.
+
+**`npm run prisma:migrate` fails with `getaddrinfo ENOTFOUND postgres`.** You
+are running Prisma on your host while `DATABASE_URL` still points at the
+Docker-only `postgres` hostname. Use
+`postgresql://trood:trood@localhost:5432/trood_leader`, start Postgres with
+`docker compose up -d postgres`, then re-run the migrate command.
+
+**Port 3000 or 3001 already in use.** The API uses `PORT` from `.env`
+(default 3000); the embedding server hard-codes 3001. Free the port or
+change the API's `PORT` (the embedding server would need a small code edit
+at `embedding-server/index.js` to honour a different port).
+
+**Schema drift after a merge.** Run `npm run prisma:generate` after pulling
+— the generated client lives under `node_modules/.prisma` and is not
+checked in, so rebasing or switching branches often leaves it stale.
+
+## Where to put changes
+
+- API routes: `src/` (Express + TypeScript)
+- Prisma schema + migrations: `prisma/`
+- Embedding mock: `embedding-server/`
+- Dev/CI config: root (`tsconfig.json`, `docker-compose.yml`, `Dockerfile`)
+
+Before opening a PR, make sure:
+
+1. `npm run build` succeeds (TypeScript compiles cleanly)
+2. Prisma migrations you added apply cleanly from an empty database
+3. Your change has a one-line note in the PR description about which
+   endpoints / tables it touches, so reviewers can spot reply-tracking or
+   classification regressions early