D1.2c Retire the datamailer client, the data app and the studio datamailer views

Split out of the combined D1.2 mail-adoption issue (community-base plan phase 1). Depends on #370 (D1.2b, landed).

Scope:

• Delete course_management/datamailer* (the client package, its payloads, sync modules, templates registry), email_app/ (donor PendingUnsubscribe, replay jobs, relay links), the data app (DatamailerOutboxEvent, DatamailerSendAudit, dispatch runs), and studio_courses/views/datamailer*.py with their templates. Studio email pages come from the package.
• P6 second PR: delete the donor models kept read-only since D1.2a/D1.2b; migrations drop their tables. Migration rehearsal with counts (P14) before landing; development-copy counts recorded in the PR.
• Link bridge URLs keep their exact paths; RELAY_LINK_BRIDGE_* settings become COMMUNITY_BASE keys declared by the mail app.
• Retire the retained datamailer outbox storage from D1.2b (P6 step 4): storage goes read-only-to-dropped with the rollback window closed by the D1.2a/D1.2b dev deploys.

Non-goals:

• No Relay-side changes; the R1.x catalog/callback contracts stay as pinned.
• No new mail purposes; the five D1.2b purposes are unchanged.

Verification:

• grep -rln datamailer --include=*.py . | grep -v migrations -> nothing.
• Open pixel and click bridge respond as before (existing contract tests).
• Fresh-database and development-copy migrations; table-drop rehearsal with counts.
• Full gate set: ruff check/format, makemigrations --check, mypy, Django suites per the change-selection plan.

Not run here: production freeze activities (D1.3, separate issue).

Start with course_management/datamailer*, email_app/, the data app, and studio_courses/views/datamailer*.py plus their templates; review the D1.2a/D1.2b context and #370 first. Use the grep check and existing contract tests, then rehearse fresh-database and development-copy migrations with table-drop counts. Done means the listed components and retained outbox storage are retired while bridge paths and full gate checks remain valid.