bggpipe init: guided, idempotent first-run setup

One command replaces the clone-era checklist: creates photos/ and
data/, writes a default config.toml, prompts for the four credentials
with hidden input (appended to a 0600 .env, only the missing ones,
values never echoed), and offers the one-time Chromium download.
Re-runs report status and fill gaps; without a TTY it reports instead
of hanging. Groundwork for any future publishing path — PyPI or a
bundled app both need exactly this wizard.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -39,11 +39,12 @@ Every stage is idempotent and resumable: kill it mid-run, restart, lose nothing.
 ```sh
 git clone https://git.kestrelsnest.social/eric/bggpipe.git
 cd bggpipe
-uv sync                                # installs Python deps
-uv run playwright install chromium     # browser for the upload stage
-cp .env.example .env                   # then fill in your keys
+uv sync              # installs Python deps
+uv run bggpipe init  # guided setup: folders, credentials, browser download
 ```
 
+The `init` wizard is idempotent — re-run it anytime to check status or add keys you skipped. It prompts for the credentials below (hidden input, saved to a `.env` it creates with owner-only permissions) and offers the one-time Playwright Chromium download. Prefer doing it by hand? `cp .env.example .env`, fill it in, and run `uv run playwright install chromium` yourself.
+
 Secrets live in environment variables only, never in config files, code, or logs. `.env` is gitignored. If you use [direnv](https://direnv.net/), the committed `.envrc` loads `.env` automatically after a one-time `direnv allow`; otherwise export the variables yourself (e.g. `set -a; source .env; set +a`).
 
 | Variable | Used by | What it is |