fix(ci): add --threads:on to all nim c commands

Commit 6aaabb5 added std/locks usage in vector/engine.nim (HNSW thread-safety).
std/locks requires --threads:on. Local builds worked because /etc/nim/nim.cfg
had threads:on, but GitHub Actions runners (setup-nim-action) compile without
threads by default, causing the test job to fail silently at runtime.

Also updated baradadb.nimble tasks for consistency.

Fixes GitHub Actions CI failure on test / clients-ci workflows.

.github/workflows/clients-ci.yml

@@ -35,8 +35,8 @@ jobs:
 
       - name: Build binaries
         run: |
-          nim c -d:release --opt:speed -d:ssl -o:build/baradadb src/baradadb.nim
-          nim c -d:release -o:build/backup src/barabadb/core/backup.nim
+          nim c -d:release --opt:speed -d:ssl --threads:on -o:build/baradadb src/baradadb.nim
+          nim c -d:release --threads:on -o:build/backup src/barabadb/core/backup.nim
 
       - name: Build Docker image
         run: docker build -t baradb:latest .
@@ -160,15 +160,15 @@ jobs:
 
       - name: Run Nim unit tests
         working-directory: clients/nim
-        run: nim c --path:src -r tests/test_client.nim
+        run: nim c --threads:on --path:src -r tests/test_client.nim
 
       - name: Run Nim integration tests
         working-directory: clients/nim
-        run: nim c --path:src -r tests/test_integration.nim
+        run: nim c --threads:on --path:src -r tests/test_integration.nim
 
       - name: Run Nim example
         working-directory: clients/nim
-        run: nim c --path:src -r examples/basic.nim || true
+        run: nim c --threads:on --path:src -r examples/basic.nim || true
 
   test-rust:
     needs: build-server