chore: add pre-commit hook configuration

Local hooks for hygiene (whitespace, YAML, merge conflicts, large files),
Go toolchain (fmt, vet, mod tidy, build), and golangci-lint matching CI.

backend/.pre-commit-config.yaml

@@ -0,0 +1,23 @@
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v5.0.0
+    hooks:
+      - id: trailing-whitespace
+      - id: end-of-file-fixer
+      - id: check-yaml
+        exclude: ^deploy/helm/templates/
+      - id: check-merge-conflict
+      - id: check-added-large-files
+
+  - repo: https://github.com/golangci/golangci-lint
+    rev: v2.1.6
+    hooks:
+      - id: golangci-lint
+
+  - repo: https://github.com/tekwizely/pre-commit-golang
+    rev: v1.0.0-rc.1
+    hooks:
+      - id: go-fmt-repo
+      - id: go-vet-repo-mod
+      - id: go-mod-tidy
+      - id: go-build-mod