fix: add golangci-lint config and fix all lint errors

- Add .golangci.yml with v2 config (errcheck, govet, staticcheck, misspell, etc.)
- Fix 32 errcheck issues across config, discord, ntfy, server packages
- Fix misspelling: cancelled → canceled
- Fix staticcheck: use append(slice...) instead of loop
- Fix staticcheck: remove empty error branch
- Use t.Setenv instead of os.Setenv/Unsetenv in tests
- Update CI workflow: add lint job, release tagging, ntfy notifications

.golangci.yml

@@ -0,0 +1,31 @@
+version: "2"
+
+run:
+  timeout: 5m
+  modules-download-mode: readonly
+
+linters:
+  enable:
+    - errcheck
+    - govet
+    - ineffassign
+    - staticcheck
+    - unused
+    - misspell
+    - unconvert
+    - bodyclose
+    - nilerr
+  settings:
+    errcheck:
+      check-type-assertions: true
+    misspell:
+      locale: US
+
+formatters:
+  enable:
+    - gofmt
+    - goimports
+
+issues:
+  max-issues-per-linter: 50
+  max-same-issues: 10