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/lib/snippet-actions.ts

@@ -155,10 +155,16 @@ export async function deleteSnippetAction(formData: FormData) {
 
   const scope = formData.get("scope") as "project" | "user" | null;
   const projectRaw = (formData.get("project") as string | null)?.trim();
+  const rawVersion = formData.get("version");
+  const version =
+    typeof rawVersion === "string" && rawVersion.length > 0
+      ? Number.parseInt(rawVersion, 10)
+      : undefined;
   const payload = {
     name: String(formData.get("name") ?? "").trim(),
     scope: scope ?? undefined,
     project: scope === "project" ? projectRaw || undefined : undefined,
+    version: Number.isFinite(version) ? version : undefined,
   };
 
   const parsed = SnippetDeleteInput.safeParse(payload);
@@ -171,6 +177,7 @@ export async function deleteSnippetAction(formData: FormData) {
     scope: parsed.data.scope,
     projectKey: parsed.data.project,
     groupNames,
+    version: parsed.data.version,
   });
   if (!deleted) throw new Error("not found");