Remediate all lint findings under the canonical golangci-lint config

Fix every finding surfaced by the canonical .golangci.yml with
golangci-lint v2.12.2 (refs #61), behavior-preserving throughout:

- err113: dynamic errors replaced with package-level sentinels and %w
  wrapping; direct comparisons converted to errors.Is
- goprintffuncname: printf-style helpers renamed with an f suffix
  (ui.Writer message methods, cli.ReportErrorf, database.Fatalf,
  vaultik stdoutf) and all call sites updated
- revive: stuttering type names renamed (blob.Handler, blob.WithReader,
  blob.ChunkPosition, storage.URL, storage.Info), doc comments added,
  unused parameters blanked, package comments added
- contextcheck/noctx: ctx threaded through blob.Packer
  (AddChunk/Flush/FinalizeBlob/PackChunks) and scanner call sites;
  context-aware exec and sql variants used
- funlen/cyclop/gocognit/nestif/dupl: oversized or duplicated
  functions split into focused helpers across production and test code
- paralleltest/tparallel/thelper/usetesting/testpackage: tests
  parallelized where safe (global log.Initialize kept in the serial
  phase), helpers marked, t.TempDir adopted, external test packages
  where only exported API is used
- gosec: integer conversions clamped or justified, header timeouts
  added, remaining findings suppressed with per-site justifications
- mnd/goconst/lll/wsl_v5/nlreturn/noinlineerr/errcheck and other
  mechanical findings fixed directly

Remove the deprecated log.LogOptions alias (callers migrated to
log.Options). make check is green.

internal/database/chunks.go

@@ -3,20 +3,25 @@ package database
 import (
 	"context"
 	"database/sql"
+	"errors"
 	"fmt"
 	"strings"
 
 	"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)
@@ -38,6 +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
@@ -52,8 +59,8 @@ func (r *ChunkRepository) GetByHash(ctx context.Context, hash string) (*Chunk, e
 		&chunk.Size,
 	)
 
-	if err == sql.ErrNoRows {
-		return nil, nil
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, nil //nolint:nilnil // nil,nil signals not-found; callers check nil
 	}
 
 	if err != nil {
@@ -63,7 +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
 	}
@@ -87,7 +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"
 
@@ -116,7 +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