Port custom emoji + sticker tagging to staging (from refactor #2362)

[kolacheee]

Summary

Port custom emoji + sticker tagging for gallery images to the staging (Node.js) codebase. Shipped to refactor in #2362; not present in staging.

Why

Lets users tag any gallery image (character / persona / global) as a custom emoji or sticker, give it a name, and size-gate it (emoji ≤256×256, sticker ≤512×512). This is the data-model layer the Conversation custom emoji/sticker feature is built on (original feature umbrella #2360, closed).

Scope (Node re-implementation)

• shared: customKind (emoji|sticker) + customName on the gallery image model + schemas; shared validation lib (size gate, mutual exclusivity).
• server: persist the new columns; tag/untag support.
• client: tag/untag UI in all three galleries (character, persona, global), name field with per-gallery uniqueness, size-gate feedback.
• Note: re-implementation on the Node stack; refactor's Rust plumbing does not carry over.

Dependencies

Depends on the persona + global galleries port — #2575 (source: refactor #2361). Tag UI spans all three galleries.

Reference

• Source: refactor Add custom emoji and sticker tagging to galleries #2362 — "Add custom emoji and sticker tagging to galleries"
• Related: original feature umbrella [Feature]: Custom Stickers + Emojis and Emoji Reacts #2360 (closed)