fix: unify DATA_DIR default to /var/lib/webhooker for all environments

Remove devDataDir() XDG-based logic. Both dev and prod now default
DATA_DIR to /var/lib/webhooker. Update Dockerfile and README to match.

README.md

@@ -59,7 +59,6 @@ or `prod` (default: `dev`). The setting controls several behaviors:
 
 | Behavior              | `dev`                            | `prod`                          |
 | --------------------- | -------------------------------- | ------------------------------- |
-| Default `DATA_DIR`    | `$XDG_DATA_HOME/webhooker` (or `$HOME/.local/share/webhooker`) | `/data` |
 | CORS                  | Allows any origin (`*`)          | Disabled (no-op)                |
 | Session cookie Secure | `false` (works over plain HTTP)  | `true` (requires HTTPS)         |
 
@@ -71,7 +70,7 @@ TTY detection, and security headers are always applied.
 | ----------------------- | ----------------------------------- | -------- |
 | `WEBHOOKER_ENVIRONMENT` | `dev` or `prod`                     | `dev`    |
 | `PORT`                  | HTTP listen port                    | `8080`   |
-| `DATA_DIR`              | Directory for all SQLite databases  | `$XDG_DATA_HOME/webhooker` (dev) / `/data` (prod) |
+| `DATA_DIR`              | Directory for all SQLite databases  | `/var/lib/webhooker` |
 | `DEBUG`                 | Enable debug logging                | `false`  |
 | `METRICS_USERNAME`      | Basic auth username for `/metrics`  | `""`     |
 | `METRICS_PASSWORD`      | Basic auth password for `/metrics`  | `""`     |
@@ -90,16 +89,16 @@ is only displayed once.
 ```bash
 docker run -d \
   -p 8080:8080 \
-  -v /path/to/data:/data \
+  -v /path/to/data:/var/lib/webhooker \
   -e WEBHOOKER_ENVIRONMENT=prod \
   webhooker:latest
 ```
 
 The container runs as a non-root user (`webhooker`, UID 1000), exposes
 port 8080, and includes a health check against
-`/.well-known/healthcheck`. The `/data` volume holds all SQLite
-databases: the main application database (`webhooker.db`) and the
-per-webhook event databases (`events-{uuid}.db`). Mount this as a
+`/.well-known/healthcheck`. The `/var/lib/webhooker` volume holds all
+SQLite databases: the main application database (`webhooker.db`) and
+the per-webhook event databases (`events-{uuid}.db`). Mount this as a
 persistent volume to preserve data across container restarts.
 
 ## Rationale
@@ -845,8 +844,8 @@ The Dockerfile uses a multi-stage build:
    golangci-lint, downloads dependencies, copies source, runs `make
    check` (format verification, linting, tests, compilation).
 2. **Runtime stage** (`alpine:3.21`) — copies the binary, creates the
-   `/data` directory for all SQLite databases, runs as non-root user,
-   exposes port 8080, includes a health check.
+   `/var/lib/webhooker` directory for all SQLite databases, runs as
+   non-root user, exposes port 8080, includes a health check.
 
 The builder uses Debian rather than Alpine because GORM's SQLite
 dialect pulls in CGO-dependent headers at compile time. The runtime