Add agent project guidance

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AGENTS.md

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+# AGENTS.md
+
+Bitcoin price tracker UI — Python FastAPI web application. Serves the HTML/Tailwind UI and
+proxies live price data from btcdata over WebSocket.
+
+## Build & Development Commands
+
+- `make run` — Start dev server on port 4000; creates `.venv` and installs `requirements.txt`
+  on first run (the `venv` target, keyed off `requirements.txt`)
+- `cp .env.example .env` — optional; `.env` is `-include`d, so a missing one is not fatal
+- `make run-local` — Same, but with `BTCDATA_URL=$BTCDATA_LOCAL_URL`
+- `make tailwind` — Watch Tailwind CSS for changes
+- `make test` — Installs test-only dependencies from `requirements-dev.txt` and runs `pytest tests/`
+- `make venv` — Bootstrap the virtualenv without starting the server
+
+## Docker
+
+Docker runs alongside local dev on different ports:
+
+| Service  | Local dev      | Docker                               |
+|----------|----------------|--------------------------------------|
+| btcprice | localhost:4000 | localhost:4041 (container port 4040) |
+| btcdata  | localhost:4100 | localhost:4102                       |
+
+`docker compose up -d` here starts **btcprice only**. Both projects attach to the external
+`btc-network` (`docker network create btc-network` once), and btcdata must be started from
+`../btcdata`. Inside the network btcprice reaches it at `http://btcdata:${DOCKER_BTCDATA_PORT:-4102}`.
+
+The image does not set `BTCDATA_URL`; deployments must provide it. `BUILD_SHA` is embedded as
+`APP_VERSION` and exposed at `/api/version`. The port in `CMD` is hardcoded to 4040.
+
+## CI/CD and production
+
+`.gitea/workflows/build.yml` tests pushes to `main` and `v*` tags on the `anvil` Gitea Actions
+runner, then publishes immutable linux/amd64 images to
+`harbor.pmagnus.cloud/btcprice/btcprice`. It embeds the short Git SHA as `APP_VERSION`.
+
+Flux image automation updates the manifests in the separate `magnus/deploy` repository and Flux
+reconciles k3s. CI has no Kubernetes credentials. Production runs two replicas—one on hostinger and
+one on citadel—and is served at https://btcprice.k8s.pmagnus.cloud through ingress-nginx and
+hostinger Traefik TLS. Deployment changes belong in `../deploy/apps/btcprice`; do not deploy with
+Compose or manual `kubectl apply`.
+
+## Legacy deployment
+
+`./deploy-citadel.sh` — build locally for the remote's arch, `docker save | ssh docker load`,
+`docker compose up -d --no-build` in `/home/pmp/apps/btcprice` on citadel (192.168.50.195).
+Mirrors `../elprice/deploy-citadel.sh`; no registry.
+
+- `docker-compose.citadel.yml` → remote `docker-compose.yml`; `.env.citadel` → remote `.env`.
+- btcdata also runs on citadel, so both attach to the external `btc-network` there and
+  `BTCDATA_URL` is `http://btcdata:4102` (service name, not a LAN address). The deploy script
+  creates the network on the remote if it is missing.
+- Temporary migration fallback at http://192.168.50.195:4041; it is not the production endpoint.
+
+## Architecture
+
+- **Framework:** FastAPI
+- **Server:** Uvicorn
+- **Templates:** Inline HTML strings + HTMX (WebSocket extension)
+- **Styling:** Tailwind CSS v4
+- **Data backend:** btcdata (separate service at `BTCDATA_URL`, default `http://localhost:4100`)
+- **No database** — all data comes from btcdata
+- **Probes:** `/api/health` is process-only liveness; `/api/ready` checks btcdata with a 3-second
+  timeout; `/api/version` exposes the build identifier
+
+### btcdata connection
+
+The browser connects only to btcprice: `ws-connect="/ws/price"` and `ws-connect="/ws/strike"`.
+Each endpoint runs `_ws_proxy`, which opens an upstream WebSocket to btcdata's `/api/price/ws`
+and `/api/strike/ws`, converts each JSON message to an HTML fragment (`price_json_to_html`,
+`strike_json_to_html`) and forwards it. `BTCDATA_URL` is the single point of contact;
+`_btcdata_ws_url` rewrites its scheme to `ws`/`wss`.
+
+## Source Layout
+
+btcprice acts as a WebSocket proxy between the browser and btcdata:
+1. Browser connects via HTMX WebSocket to `/ws/price` or `/ws/strike`
+2. btcprice proxies to btcdata's `/api/price/ws` or `/api/strike/ws`
+3. JSON from btcdata is transformed into HTML fragments (in `price_json_to_html` / `strike_json_to_html`)
+4. HTMX swaps the HTML into the DOM by matching element IDs (`price-display`, `strike-display`)
+
+### Key Conventions
+
+- Numbers use Danish locale formatting (dots for thousands, commas for decimals) via `da_fmt()`
+- Price changes are color-coded: green (up), red (down), gray (unchanged)
+- All HTML is built as string concatenation in Python — no template engine