relay: NIP-09 a-tag deletion of kind:30620 (workflow def) leaves the events row alive

## Summary

`handle_a_tag_deletion` in `crates/sprout-relay/src/handlers/side_effects.rs` calls `state.db.delete_workflow(...)` for the `KIND_WORKFLOW_DEF` branch. `delete_workflow` (in `crates/sprout-db/src/workflow.rs:451`) only `DELETE`s from the `workflows` table (with `workflow_runs` / `workflow_approvals` cascading). It does **not** touch the underlying kind:30620 event row in `events` — that row stays alive with `deleted_at IS NULL`, so REQ subscribers for kind:30620 events keep getting the deleted workflow's authoring event back.

This is the same bug shape as #714 (a-tag NIP-09 deletion no-op for kind:30023), just localized to the workflow kind. #714 fixes all *other* parameterized-replaceable kinds; the workflow branch was intentionally left as-is in that PR to avoid silently changing workflow lifecycle assumptions.

## Why this is its own issue

The workflow team likely has assumptions baked in about whether the events row sticks around (e.g. for "what was the workflow definition that just got deleted" audit lookups via `get_event_by_id_including_deleted`, or REQ-based discovery of historical defs). Closing this gap requires either:

1. Calling `Db::soft_delete_by_coordinate(KIND_WORKFLOW_DEF as i32, &pubkey, d_tag)` inside the workflow branch in addition to `delete_workflow`, **and** verifying nothing else relies on the row staying live; OR
2. Deciding that's a behavior change the workflow team explicitly wants — and at that point we might also want to revisit whether `delete_workflow` should be invoked at all on a-tag deletion vs e-tag deletion, since the two paths currently diverge.

Either way, this needs eyes from someone who knows the workflow code, which is why it's scoped out of #714 / #716.

## Reproduce

1. Publish a kind:30620 workflow definition.
2. Publish a kind:5 deletion with `["a", "30620::"]`.
3. The workflow is removed from `workflows` (good).
4. `REQ { kinds: [30620], authors: [] }` still returns the kind:30620 event (bug).

## Code pointers

- `crates/sprout-relay/src/handlers/side_effects.rs:~1330` — the workflow branch in `handle_a_tag_deletion`.
- `crates/sprout-db/src/workflow.rs:451` — `delete_workflow`, only touches `workflows`.
- `crates/sprout-db/src/event.rs` — `soft_delete_by_coordinate` (will exist post-#716; the helper to call).

## Out of scope

Whether to keep `delete_workflow`'s side effects (workflow_runs cascade, etc.) intact while also soft-deleting the events row. That's the workflow team's call.

## Discovered while

Building #716. Caught by tracing the existing match arm; flagged in the PR description there too.