Centralize replaceable event persistence (#6660)

## Why
Command persistence duplicated NIP-33 replacement SQL in the relay and
obscured the boundary between database runtime concerns and domain-store
behavior. This proving slice establishes that boundary inside the
existing `buzz-db` crate.

## What
- Centralize parameterized-replaceable coordinate locking, ordering,
replacement, watermark, and mention-indexing behavior in `buzz-db`
- Expose transaction-required replacement and ordinary-insertion seams
while keeping transaction ownership and workflow conflict messages in
the relay
- Cover concurrency, same-second ties, stale writes, replay, caller
rollback, nested rollback recovery, and mention-index atomicity with
focused PostgreSQL tests

## Risk Assessment
Medium — this changes core event persistence and command idempotency
paths, while intentionally preserving NIP-33, NIP-RS, mesh, and workflow
conflict semantics.

## References
- Architecture guardrail:
https://github.com/TheSentinel454/buzz/issues/34
- Proving slice: https://github.com/TheSentinel454/buzz/issues/3,
https://github.com/TheSentinel454/buzz/issues/4,
https://github.com/TheSentinel454/buzz/issues/6

Generated with Codex

---------

Signed-off-by: tornquist <tornquist@squareup.com>

.github/workflows/ci.yml

@@ -679,6 +679,19 @@ jobs:
           VALUES ('00000000-0000-4000-8000-00000000c0de', 'localhost:3000')
           ON CONFLICT (lower(host)) DO NOTHING
           ;"
+      - name: Replaceable persistence PostgreSQL tests
+        # Transaction, concurrency, and mention-index coverage for the
+        # replaceable-event store seam. These tests require real Postgres and
+        # are ignored by the infrastructure-free unit-test job.
+        run: |
+          filter='package(buzz-db) and test(/tests::(parameterized_|concurrent_parameterized_)/)'
+          cargo nextest run \
+            --archive-file target/ci/backend-integration-tests.tar.zst \
+            -E "${filter}" \
+            --run-ignored ignored-only
+        env:
+          DATABASE_URL: postgres://buzz:${{ env.BUZZ_TEST_POSTGRES_PASSWORD }}@localhost:5432/buzz
+          TEST_DATABASE_URL: postgres://buzz:${{ env.BUZZ_TEST_POSTGRES_PASSWORD }}@localhost:5432/buzz
       - name: Start relay
         run: |
           chmod +x ./target/ci/buzz-relay