feat: add timeout handling for cache database operations

[knqyf263]

Description

This PR adds timeout handling to the cache database operations to prevent Trivy from hanging indefinitely when another process has locked the database.

Changes:

• Add 5-second timeout for BoltDB cache operations
• Return clearer error messages:
  • Timeout errors: "cache may be in use by another process"
  • Other errors: "unable to open cache DB"
• Update troubleshooting documentation with lock error guidance

Before:

When another process locks the cache database, Trivy hangs indefinitely with no output.

After:

$ trivy image alpine:latest
2025-08-04T12:00:00.000+0000    FATAL   cache may be in use by another process: timeout

Related issues

• Fixes cases where Trivy hangs indefinitely when another process has locked the cache database

Related PRs

• feat: add timeout to BoltDB open to prevent indefinite waiting trivy-db#560

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[Copilot]

Pull Request Overview

This PR adds timeout handling to cache database operations to prevent Trivy from hanging indefinitely when the cache database is locked by another process.

• Adds a 5-second timeout to BoltDB cache operations with clearer error messages
• Updates error messages to distinguish between timeout and other database errors
• Enhances troubleshooting documentation with comprehensive guidance on database lock issues

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
pkg/cache/fs.go	Adds timeout configuration and improved error handling for cache database operations
go.mod	Updates trivy-db dependency version
docs/docs/references/troubleshooting.md	Replaces brief parallel execution note with comprehensive database lock troubleshooting guide

Comments suppressed due to low confidence (1)

pkg/cache/fs.go:17

• [nitpick] The constant name 'defaultFSCacheTimeout' could be more descriptive. Consider renaming it to 'defaultCacheDBTimeout' or 'defaultBoltDBTimeout' to better reflect that it's specifically for database operations.

const defaultFSCacheTimeout = 5 * time.Second

[DmitriyLewen]

LGTM