feat(grz-db): add cases table and pluggable case resolver

[Hoeze]

Introduces case tracking so multiple submissions can be grouped into one case (an initial submission plus its addition/correction/followup for the same patient/case).

The authoritative case identity is the RKI pseudonym (psn), unique once assigned; (submitter_id, local_case_id) are non-unique resolution keys used until a pseudonym exists. Case lookup is a pluggable strategy (SubmitterLocalCaseResolver today, PsnResolver for the future PSN-based flow). submitter_id stays on submissions.

feat(grzctl): add db case commands and link submissions to cases on populate

Adds grzctl db case (list/show/modify/create/relink/delete) and links each submission to its case during db submission populate, enforcing that a new case starts with an initial (or test) submission and that at most one initial exists per case. A new Alembic migration adds the cases table and a nullable submissions.case_id, backfills existing submissions by grouping on (submitter_id, pseudonym), and keeps submitter_id on submissions. Deploying requires grzctl db upgrade.

Fixes #632