enhance: Make Drug names unique & handle defining drugs that already exist (gracefully)

feat: 303 if drug name already exists

When creating a new drug with POST /drugs/, if a drug with the same name already exists, the edihkal API will respond with a 303 See Other redirect pointing the client towards the drug that already exists.

build(srv): Add sea-orm-postgres feature flag

Adds feature flags to edihkal for selecting which database backend is used by sea-orm.