feat(cli): add URL prefix fix command

[Octobug]

go run cmd/answer/main.go fix --help
Fix stored URL prefixes for branding, avatars, and post content.

WARNING: Stop Answer and back up your database before running this command.

Rows already using DST_PREFIX are skipped so the command can be rerun safely.
If a text field contains both prefixes, it will be skipped and reported.

After fixing branding data, the related site info cache will be invalidated.

Usage:
  answer fix [all|branding|avatar|post] SRC_PREFIX DST_PREFIX [flags]

Examples:
  # Preview what would change
  answer fix all /uploads/ /uploads/new/ --dry-run

  # Fix up to 50 rows
  answer fix all /uploads/ /cdn/ --limit 50

  # Fix only avatars
  answer fix avatar /uploads/ /cdn/

Flags:
      --dry-run     show what would be changed without modifying the database
  -h, --help        help for fix
      --limit int   stop after fixing NUM rows total across all selected fix types (0 = unlimited)

Global Flags:
  -C, --data-path string   data path, eg: -C ./data/ (default "/data/")

I've tested locally with fake data generated in sqlite db, including:

• 50,000 question
• 50,000 revision
• 250,000 answer
• 250,000 file records

It took about 10 minutes to complete the fixing.

[Copilot]

Pull request overview

Adds a new answer fix CLI command to rewrite stored URL prefixes across branding assets, user avatars, and post content (including revisions and file records), with support for dry-run and a global row limit. This fits into the codebase as an administrative maintenance tool for safely migrating stored paths (e.g., /uploads/ → /cdn/) while Answer is stopped.

Changes:

• Introduces URL-prefix classification and batched DB rewrite logic for branding, avatars, posts/revisions, and file_record URLs.
• Adds cache invalidation for branding site-info when using file-backed cache storage.
• Wires a new Cobra command (answer fix ...) with --dry-run and --limit flags.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/cli/fix.go	Core fix orchestration, prefix classification/replacement utilities, batching, and summary output.
internal/cli/fix_post.go	Applies prefix fixes to question/answer bodies, revisions, and post-related file records.
internal/cli/fix_filerecord.go	Updates file_record.file_url for selected sources.
internal/cli/fix_cache.go	Invalidates branding site-info cache in file-backed memory cache.
internal/cli/fix_branding.go	Fixes branding JSON fields in site_info and related branding file records.
internal/cli/fix_avatar.go	Fixes custom avatar URLs in user avatar JSON and related avatar file records.
cmd/command.go	Adds the fix command and its flags to the CLI.

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