bump to 0.3.1+v5.1.0

Add env vars introduced upstream across v4.6 → v5.0:
- CONVERSION_UPLOAD_ENABLED (v4.8.5) — recipe override true (upstream default false)
- CONVERSION_FILE_MAX_SIZE / _EXTENSIONS_ALLOWED / _API_* (v4.5.0 source)
- DB_PSYCOPG_POOL_ENABLED (v4.8.1)
- DJANGO_EMAIL_URL_APP (v4.6.0) — recipe sets to https://${DOMAIN}
- MEDIA_AUTH_ORIGINAL_URL_HEADER (v5.0.0)

All except CONVERSION_UPLOAD_ENABLED and DJANGO_EMAIL_URL_APP use the
bare-key defer-to-upstream pattern (like existing LOGGING_LEVEL_*), so
upstream defaults apply unless the operator sets them in .env.

Document all 10 vars (commented) in .env.sample.

.env.sample

@@ -72,3 +72,37 @@ LOGGING_LEVEL_LOGGERS_APP=INFO
 ##############################################################################
 # Set to false to disable automatic migrations on backend startup
 # AUTO_MIGRATIONS=true
+
+##############################################################################
+# CONVERSION (file upload → Yjs via y-provider)
+##############################################################################
+# Allow uploading .docx/.md files for conversion to Yjs.
+# Recipe sets this to true (upstream default is false).
+# CONVERSION_UPLOAD_ENABLED=true
+# Upstream defaults (uncomment to override):
+# CONVERSION_FILE_MAX_SIZE=20971520
+# CONVERSION_FILE_EXTENSIONS_ALLOWED=.docx,.md
+# CONVERSION_API_ENDPOINT=convert
+# CONVERSION_API_CONTENT_FIELD=content
+# CONVERSION_API_TIMEOUT=30
+# CONVERSION_API_SECURE=false
+
+##############################################################################
+# DATABASE CONNECTION POOL
+##############################################################################
+# Upstream default (uncomment to override):
+# DB_PSYCOPG_POOL_ENABLED=false
+
+##############################################################################
+# EMAIL APP URL
+##############################################################################
+# DJANGO_EMAIL_URL_APP is set automatically in compose.yml to https://${DOMAIN}.
+# Override is not exposed via .env — edit compose.yml directly if needed.
+# (Upstream default falls back to Django's Site framework, not configured here.)
+
+##############################################################################
+# MEDIA AUTH HEADER
+##############################################################################
+# Header the backend reads for media auth subrequests.
+# Upstream default matches the recipe's nginx config (X-Original-URL).
+# MEDIA_AUTH_ORIGINAL_URL_HEADER=HTTP_X_ORIGINAL_URL