Update golangci-lint to v2.12.2 with canonical config (#62)

Updates golangci-lint to v2.12.2 everywhere it is pinned and installs the canonical `.golangci.yml`, then remediates every finding the new linter/config surfaces so `make check` is green.

## Version bump

- `Dockerfile` lint stage: `golangci/golangci-lint:v2.11.3-alpine` -> `v2.12.2-alpine` (digest-pinned, date comment updated)
- `Makefile` `deps` target: `go install` moved from the old v1 module path at `@latest` to the pinned `github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2`
- `.golangci.yml` replaced with the canonical config (v2 schema; settings under `linters.settings` so the thresholds actually apply; `default: all` with the standard six disables)
- `script/bootstrap` installs golangci-lint via the system package manager and carries no version pin, so it is unchanged
- CI (`.gitea/workflows/check.yml`) only runs `script/cibuild`, so it needed no change

## Lint remediation

The canonical config surfaced ~3,300 findings across 56k lines. All are fixed, behavior-preserving; incorporates and supersedes the per-package mechanical passes already merged to `main` (refs #61). Highlights:

- `err113`: dynamic errors replaced with package sentinels + `%w` wrapping; comparisons via `errors.Is`
- `goprintffuncname`: printf-style helpers renamed with an `f` suffix (`ui.Writer` message methods, `cli.ReportErrorf`, `database.Fatalf`) and all call sites updated
- `revive` stutter renames: `blob.Handler`, `blob.WithReader`, `blob.ChunkPosition`, `storage.URL`, `storage.Info`; missing doc comments added
- `contextcheck`/`noctx`: `context.Context` threaded through `blob.Packer` and the scanner call sites; context-aware `exec`/`sql` variants
- `funlen`/`cyclop`/`gocognit`/`dupl`: oversized and duplicated functions split into focused helpers (production and test code)
- tests: `t.Parallel()` added where safe (global logger init kept in the serial phase for `-race`), `t.TempDir()`/`t.Helper()` adopted, several suites converted to external test packages
- `gosec`: bounded integer conversions, `ReadHeaderTimeout` on the test HTTP server; remaining warnings suppressed per-site with justifications
- remaining `nolint` directives are rare, targeted, and each carries a reason (e.g. `nilnil` not-found contract in the repository layer, fx module globals, on-disk snake_case struct tags)
- removed the deprecated `log.LogOptions` alias (callers migrated to `log.Options`)

`make check` (tests with `-race`, lint, fmt-check) passes.

Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #62
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/database/chunks.go

@@ -10,14 +10,18 @@ import (
 	"sneak.berlin/go/vaultik/internal/log"
 )
 
+// ChunkRepository provides access to the chunks table, which tracks
+// content-defined chunks by hash and size.
 type ChunkRepository struct {
 	db *DB
 }
 
+// NewChunkRepository creates a ChunkRepository backed by db.
 func NewChunkRepository(db *DB) *ChunkRepository {
 	return &ChunkRepository{db: db}
 }
 
+// Create inserts a chunk row (idempotently), using tx when non-nil.
 func (r *ChunkRepository) Create(ctx context.Context, tx *sql.Tx, chunk *Chunk) error {
 	query := `
 		INSERT INTO chunks (chunk_hash, size)
@@ -39,6 +43,8 @@ func (r *ChunkRepository) Create(ctx context.Context, tx *sql.Tx, chunk *Chunk)
 	return nil
 }
 
+// GetByHash returns the chunk with the given hash, or nil if it is not
+// known to the index.
 func (r *ChunkRepository) GetByHash(ctx context.Context, hash string) (*Chunk, error) {
 	query := `
 		SELECT chunk_hash, size
@@ -54,7 +60,7 @@ func (r *ChunkRepository) GetByHash(ctx context.Context, hash string) (*Chunk, e
 	)
 
 	if errors.Is(err, sql.ErrNoRows) {
-		return nil, nil
+		return nil, nil //nolint:nilnil // nil,nil signals not-found; callers check nil
 	}
 
 	if err != nil {
@@ -64,7 +70,11 @@ func (r *ChunkRepository) GetByHash(ctx context.Context, hash string) (*Chunk, e
 	return &chunk, nil
 }
 
-func (r *ChunkRepository) GetByHashes(ctx context.Context, hashes []string) ([]*Chunk, error) {
+// GetByHashes returns the chunks whose hashes appear in hashes, ordered by
+// chunk hash. Unknown hashes are silently omitted from the result.
+func (r *ChunkRepository) GetByHashes(
+	ctx context.Context, hashes []string,
+) ([]*Chunk, error) {
 	if len(hashes) == 0 {
 		return nil, nil
 	}
@@ -88,7 +98,7 @@ func (r *ChunkRepository) GetByHashes(ctx context.Context, hashes []string) ([]*
 		args[i] = hash
 	}
 
-	query += querySb75.String()
+	query += querySb75.String() //nolint:gosec // G202: appends "?" placeholders only
 
 	query += ") ORDER BY chunk_hash"
 
@@ -117,7 +127,11 @@ func (r *ChunkRepository) GetByHashes(ctx context.Context, hashes []string) ([]*
 	return chunks, rows.Err()
 }
 
-func (r *ChunkRepository) ListUnpacked(ctx context.Context, limit int) ([]*Chunk, error) {
+// ListUnpacked returns up to limit chunks that are not yet stored in any
+// blob, ordered by chunk hash.
+func (r *ChunkRepository) ListUnpacked(
+	ctx context.Context, limit int,
+) ([]*Chunk, error) {
 	query := `
 		SELECT c.chunk_hash, c.size
 		FROM chunks c