storage, embeddings and backfill need sqlalchemy[asyncio] (greenlet). The
package __init__ now loads their names on first access and raises an
ImportError naming the extra when they are missing. The middleware finds a
DatabaseSink or EmbeddingSink through sys.modules instead of importing them.
Table prefix, embedding dim, sources and event text keys move to constants.py
so they import on a base install.
rsp2k