Add configurable filename templates

[jamditis]

Motivation

Surfaced by the v1.2 research pass. Downloaded filenames are currently platform/post-id based with limited structure.

Scope

• Configurable filename templates, e.g. {platform}/{username}/{postId}_{index}.
• Sane per-platform defaults; validate/sanitize template output through the existing sanitizeDownloadPath gate.

Notes

Effort: medium. CWS risk: low.

[jamditis]

Engine and wiring in #47. Two follow-ups filed: resolver item.meta support (which unblocks {postId} and {username}) and the options-page field.

One note on the scope as written. The example {platform}/{username}/{postId}_{index} spans a folder and a filename, and those are two settings here — downloadPath already templates the folder half. Rather than let a filename template create directories, which would mean loosening the traversal defense in sanitizeDownloadPath, both settings now share one token vocabulary and the filename validator rejects / with a message pointing at the folder setting. Same result, without weakening the path sanitizer.