nip94: add missing tags to FileMetadata (ox, thumb, image, summary, alt)

[PrarthanaPurohit]

Description

Completes the FileMetadata struct in nostr::nips::nip94 to match the full NIP-94 spec.

The following tags were missing from the struct and are now supported:

Tag	Field	Description
ox	original_hash	SHA-256 of the original file before any server-side transforms
thumb	thumb	Thumbnail URL
image	image	Preview image URL
summary	summary	Short text description
alt	alt	Accessibility alt text
fallback	fallback	List of fallback download URLs (one tag per URL)

Changes span three files:

• event/tag/kind.rs — adds TagKind::Ox ("ox") and TagKind::Fallback ("fallback")
• event/tag/standard.rs — adds TagStandard::OriginalHash(Sha256Hash) and TagStandard::Fallback(Url) with parsing and serialization
• nips/nip94.rs — adds the six new fields to FileMetadata, builder methods for each, and updates From<FileMetadata> for Vec<Tag> and TryFrom<Vec<Tag>> for FileMetadata for full round-trip fidelity. Three new tests cover the added fields.

Notes to the reviewers

• ox is a two-character tag so it cannot use SingleLetterTag. A dedicated TagKind::Ox variant is added following the same pattern as other named kinds (e.g. TagKind::Blurhash, TagKind::Magnet).
• fallback maps to multiple tags (one per URL) rather than a single multi-value tag, matching the NIP-94 spec. FileMetadata stores them as Vec<Url> and From emits one ["fallback", url] tag per entry; TryFrom collects all of them.
• thumb and image already existed as TagStandard::Thumb(Url, Option<ImageDimensions>) and TagStandard::Image(Url, Option<ImageDimensions>). FileMetadata stores only the Url (NIP-94 does not specify dimensions for these) and serializes with None for the dimension slot.
• This PR was motivated by real-world use in nostreye, a Raspberry Pi camera that publishes Kind 1063 events to Nostr relays via Blossom.

Checklist

• I followed the contribution guidelines
• I updated the CHANGELOG (if applicable)
• I personally wrote and understood all code in this PR

[TheAwiteb]

Hi @PrarthanaPurohit, thank you for your work, but we no longer update the tags until we done this refactor #907. it also block #1216, #1283 and #1204.

I'll make this PR a draft and you can continue work on it after #907 closed.

[yukibtc]

Replaced by #1347