Make MySQL image configurable for CPUs without x86-64-v2

mysql:8.4 (Oracle Linux 9 build) aborts with 'Fatal glibc error: CPU does
not support x86-64-v2' on generic VM CPU types (kvm64/qemu64) and pre-2009
hardware. MYSQL_IMAGE env var (default mysql:8.4) lets such hosts run the
baseline-x86-64 mysql:8.0-debian build instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -304,5 +304,6 @@ Cairo — `brew install pango` on macOS). The Docker image includes them.
 | "Could not verify group membership (group overage)" | Grant delegated `GroupMember.Read.All` + admin consent, or scope the group claim to "Groups assigned to the application". |
 | Notification warning "OBO token exchange failed" | Check `ENTRA_CLIENT_SECRET`, and that `Mail.Send` has admin consent. |
 | `api` container restarts at boot | MySQL still initializing; the entrypoint retries migrations 12×. Check `docker compose logs mysql`. |
+| mysql dies instantly: "Fatal glibc error: CPU does not support x86-64-v2" | The host/VM CPU lacks x86-64-v2 (generic `kvm64`/`qemu64` VM CPU type, or pre-2009 hardware). Either set the hypervisor CPU type to `host` and reboot the VM, or set `MYSQL_IMAGE=mysql:8.0-debian` in `.env` and `docker compose down -v && docker compose up -d`. |
 | Power BI can't authenticate | Update Connector/NET (needs `caching_sha2_password`), verify the `powerbi_ro` grants (`scripts/powerbi_grants.sql`). |
 | Uploads fail at ~25 MB | Raise `MAX_UPLOAD_MB` (API) — nginx `client_max_body_size` is 50 MB in `frontend/nginx.conf`. |