style: adopt golangci-lint v2 and resolve all findings

Add .golangci.yml (v2): the standard linters plus bodyclose, errorlint, noctx, misspell, rowserrcheck, sqlclosecheck, unconvert, usestdlibvars, with gofumpt + goimports formatters. Enable the std-error-handling exclusion preset for idiomatic Close/Remove/Setenv ignores; scope-exclude SA1019 (parser.ParseDir in tools/openapigen) and ST1005 (intentional capitalized user-facing error copy that tests assert verbatim). No inline nolint directives were introduced.

Resolve all 217 findings behavior-preserving: gofumpt/goimports formatting, explicit blank assignment on intentionally ignored errors, errors.Is/errors.As and %w wrapping, context-aware stdlib calls (CommandContext/QueryContext/NewRequestWithContext/Dialer), staticcheck simplifications, removed redundant conversions, http.StatusOK and http.MethodGet, inlined the go:fix intPtr helper, and deferred sql rows Close. Add a golangci CI job mirroring the existing Go jobs.

internal/database/migrate_data.go

@@ -67,7 +67,7 @@ func MigrateData(srcPath, dstDSN string) error {
 		return errors.New("destination DSN is required")
 	}
 
-	if err := os.MkdirAll(path.Dir(srcPath), 0755); err != nil {
+	if err := os.MkdirAll(path.Dir(srcPath), 0o755); err != nil {
 		return err
 	}
 
@@ -144,7 +144,7 @@ func ExportPostgresToSQLite(srcDSN, dstPath string) error {
 	if srcDSN == "" {
 		return errors.New("source DSN is required")
 	}
-	if err := os.MkdirAll(path.Dir(dstPath), 0755); err != nil {
+	if err := os.MkdirAll(path.Dir(dstPath), 0o755); err != nil {
 		return err
 	}
 	// Start from an empty file so AutoMigrate creates the canonical schema.