fix: pin golangci-lint install to commit SHA (fixes #13)

- Fix Dockerfile: use correct v2 module path with commit SHA
  (github.com/golangci/golangci-lint/v2/cmd/golangci-lint@eabc2638...)
- Add CGO_ENABLED=0 for alpine compatibility
- Fix 35 lint issues found by golangci-lint v2.1.6:
  - Remove unused nolint directives, add nolintlint where needed
  - Pre-allocate migrations slice
  - Use t.Context() instead of context.Background() in tests
  - Fix blank lines between comments and exported functions in ui.go

internal/models/model.go

@@ -47,7 +47,7 @@ func (b *Base) GetDB() *sql.DB {
 }
 
 // GetUserLookup returns the DB as a UserLookup if it implements the interface.
-func (b *Base) GetUserLookup() UserLookup { //nolint:ireturn
+func (b *Base) GetUserLookup() UserLookup { //nolint:ireturn,nolintlint // returns interface by design
 	if ul, ok := b.db.(UserLookup); ok {
 		return ul
 	}
@@ -56,7 +56,7 @@ func (b *Base) GetUserLookup() UserLookup { //nolint:ireturn
 }
 
 // GetChannelLookup returns the DB as a ChannelLookup if it implements the interface.
-func (b *Base) GetChannelLookup() ChannelLookup { //nolint:ireturn
+func (b *Base) GetChannelLookup() ChannelLookup { //nolint:ireturn,nolintlint // returns interface by design
 	if cl, ok := b.db.(ChannelLookup); ok {
 		return cl
 	}