fix: address Phase 4 code review (5 findings + symmetric snippet fix)

1. lib/snippets.ts + lib/memory-actions.ts (×2): shared-project key
   lookups were querying `projects` by key with no visibility scope. Since
   `projects.key` is unique per user (not global), the unscoped match
   could resolve another user's project entirely. Restricted the lookups
   to `readableProjectIds(userId, groupNames)` — own + shared only.

2. lib/access.ts getAccessibleProjects: a stray `if (existing) continue`
   inside the share-collapse loop short-circuited on the first match,
   killing the rw-beats-ro upgrade path. Two-group cases where one share
   was ro and another rw on the same project were incorrectly resolved
   as ro. Replaced with explicit owner/rw skip.

3. lib/mcp/tools.ts snippetPut: removed a dead `void exists` block that
   looked like an authorization pre-flight but was actually a no-op —
   real write authorization lives inside putSnippet, called next. Added
   a comment at the call site documenting where the check is.

4. memory.delete + snippet.delete: previously had no optimistic-lock
   CAS, so a concurrent peer edit could be silently overwritten by a
   delete on a stale view. Added optional `version` to MemoryDeleteInput
   (new) and SnippetDeleteInput (extended); UPDATE WHERE now CASes on
   version; 0-row response surfaces CONCURRENT_EDIT_ERROR. Web detail
   pages pass `version` through hidden form inputs. When the caller
   doesn't supply a version, falls back to the version we just read in
   the same handler for in-handler consistency.

5. lib/mcp/tools.ts memorySearch re-fetch: missing `isNull(deletedAt)`
   on the post-search row hydration left a TOCTOU window where a row
   soft-deleted between the search and the re-fetch would be returned.
   Visibility is still enforced by searchMemories itself.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/app/(authed)/memories/[id]/page.tsx

@@ -236,6 +236,7 @@ export default async function MemoryDetailPage({
       {!isEditing && canWrite ? (
         <form action={deleteMemoryAction} className="flex justify-end">
           <input type="hidden" name="id" value={m.id} />
+          <input type="hidden" name="version" value={m.version} />
           <Button type="submit" variant="danger" size="sm">
             Delete memory
           </Button>