Add optional filename_template support and filename migration CLI

[wrxco]

Summary

This PR adds optional filename templating for downloaded media (and enclosure paths) while preserving existing behavior by default.

What’s included

• New optional feed config key: filename_template
• Supported tokens: {{id}}, {{title}}, {{pub_date}}, {{feed_id}}
• Validation for template tokens in config loading
• Download/output naming now uses template when set
• RSS enclosure URLs reflect the configured filename format
• One-time migration tooling:
  • --migrate-filenames
  • --migrate-filenames-dry-run
• Tests and docs updates (config.toml.example, README)

Backward compatibility

• No breaking change intended.
• If filename_template is unset/empty, behavior remains legacy (ID-based naming).

Why

Many users want readable/stable filenames that include publish date/title while retaining deterministic IDs.

Example

Use this in config.toml:

[feeds.myfeed]
url = "https://www.youtube.com/playlist?list=..."
filename_template = "{{pub_date}}_{{title}}_{{id}}"

Migration

For existing downloaded media, users can migrate once:

./bin/podsync --config config.toml --migrate-filenames

Preview only:

./bin/podsync --config config.toml --migrate-filenames --migrate-filenames-dry-run

[Copilot]

Pull request overview

Adds optional, token-based filename templating for downloaded media and RSS enclosure paths, plus a CLI mode to migrate already-downloaded files from legacy ID-based names to the configured template.

Changes:

• Introduces filename_template in feed config and validates allowed tokens during config loading.
• Updates naming logic (download output + RSS enclosure URLs) to use the templated, sanitized episode filename.
• Adds --migrate-filenames / --migrate-filenames-dry-run flow and a new migration service with tests.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
services/migrate/migrate.go	Adds filename migration service to rename legacy files to templated names.
services/migrate/migrate_test.go	Tests migration behavior (rename + dry-run).
pkg/ytdl/ytdl.go	Uses new episode base/name helpers for youtube-dl output naming.
pkg/feed/xml.go	Implements templated filename rendering/sanitization and token validation; refactors extension handling.
pkg/feed/xml_test.go	Adds tests for enclosure URL formatting and template validation/name generation.
pkg/feed/config.go	Adds FilenameTemplate to feed config struct.
config.toml.example	Documents filename_template usage and example output.
cmd/podsync/main.go	Adds migration CLI flags and executes migration mode before downloader/updater.
cmd/podsync/config.go	Validates filename_template during config validation.
cmd/podsync/config_test.go	Extends config parsing tests and adds validation failure test for bad tokens.
README.md	Documents the fork feature set and the migration CLI usage.

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