Fix area generation from multipolygon input In case the waterway axis representing a stretch intersects distinct parts of the input area, a multipolygon has to be returned. Before, an arbitrary polygon was selected due to the used function not being set-returning. In passing, consistently name respective columns in schema.

# documentation
**/README*
**/docs
**/*.md

# backend binary
cmd/gemma/gemma

# Dependency directories
**/node_modules

# local dotenv environment variables file
client/.env.local

# Tags tables
**/TAGS