A gRPC service and its clients ship on their own schedules, so the schema has to change while both old and new binaries are running. Protobuf is built for exactly that: the compatibility contract lives in field numbers and wire types rather than in names, and a reader that does not recognise a field skips it instead of failing. ...
A foreign key makes a relationship enforceable during ordinary database writes. A task can reference an existing project, and the database can reject an unknown project identifier even when an import or a new application code path forgets to check it. The constraint defines referential integrity; it does not decide whether the selected project is the correct one for the business operation. ...
A migration that rewrites a busy table in one committing statement is long and blocking. A sequence of small, backward-compatible changes moves the same schema forward with no maintenance window, because each step is separately deployable and separately reversible. What is being managed is a lock, and the question is how long it is held — which depends on how much data the statement touches. ...