Proxy Hesychia Stripe webhooks off the static site (#42)

## Summary
- Closes #41.
- Stripe live endpoint `we_1TzDNHDV0RPXOyxGCOftavHV` posts to `https://hesychia.ai/api/finance/webhooks/stripe/`. On 2026-10-09 the `chat_web_app` prod vhost returned HTTP 405 fifteen times (`request_time` 0.000, User-Agent Stripe). `pending_webhooks` stayed 1.
- `try_files` falls through to `index.html`, and nginx rejects POST on that static file. The Django handler on `chat_backend` never sees the body.
- `chat_web_app` now proxies `/api/finance/webhooks/` to the same-env `chat_backend` port (`8003` prod, `8013` beta) via `host.docker.internal`. `Host` is the backend public hostname so `ALLOWED_HOSTS` matches. Other static apps are unchanged.

## Test plan
- [ ] `ansible-playbook playbooks/site.yml --syntax-check` (or the web-static portion) renders without error
- [ ] Redeploy web-static (`scripts/deploy.sh` path that runs the web-static role) so `nginx.conf` reloads
- [ ] `curl -i -X POST http://127.0.0.1:8082/api/finance/webhooks/stripe/` from an app host reaches Django (400 invalid signature or 503 missing secret), not 405
- [ ] Stripe Dashboard: resend a failed event to the Hesychia.ai endpoint and confirm 2xx

Reviewed-on: #42

roles/web-static/templates/nginx.conf.j2

@@ -30,6 +30,34 @@ server {
     root {{ (app_catalog[a.name].webroot_pattern | default(web_static_root ~ '/{env}_' ~ a.name)) | replace('{env}', a.env) }};
     index index.html;
 
+{%- set api_backend_name = app_catalog[a.name].api_backend | default('') %}
+{%- if api_backend_name %}
+{%- set api_proxy_prefix = app_catalog[a.name].api_proxy_prefix | default('/api/') %}
+{%- set api_match = namespace(port=0, ok=false) %}
+{%- for item in host_apps | default([]) %}
+{%- if item.name == api_backend_name and item.env == a.env and (item.enabled | default(true)) %}
+{%- set api_match.port = item.port %}
+{%- set api_match.ok = true %}
+{%- endif %}
+{%- endfor %}
+{%- if api_match.ok %}
+{%- set api_host = (app_catalog[api_backend_name].public_hosts | default({}))[a.env] | default('') %}
+
+    # Finance webhooks (Stripe) arrive on the public product host. The SPA
+    # try_files fallback is a static file and returns 405 for POST.
+    location {{ api_proxy_prefix }} {
+        proxy_pass http://host.docker.internal:{{ api_match.port }};
+        proxy_http_version 1.1;
+        proxy_set_header Host {{ api_host if api_host else '$host' }};
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_read_timeout 60s;
+        client_max_body_size 2m;
+    }
+{%- endif %}
+{%- endif %}
+
 {% if app_catalog[a.name].error_page_404 | default('') %}
     error_page 404 {{ app_catalog[a.name].error_page_404 }};