feat(expressions): custom emojis and stickers per space

Emojis and stickers get separate tables on purpose: an emoji sits inside a
sentence and a sticker is the whole message, so their sizes, limits and render
paths differ.

Emoji names are unique per space and restricted to letters, digits and
underscore — :name: has to resolve to one image, and a space or colon inside
the name would make the reference impossible to delimit. Sticker names are
labels picked from a grid, so they accept anything.

Rendering reuses the existing markdown pipeline by rewriting :name: into image
syntax, which needs no plugin. Code spans and fences are left alone: text
someone wrapped in backticks was meant to be shown literally, and turning it
into an image would destroy what they wrote.

Messages carry stickerId with ON DELETE set null — removing a sticker from the
space must not delete the history of everyone who used it. The server refuses a
sticker from another space, which would otherwise leak images between unrelated
servers.

Expressions load once per space and stay in memory: the message renderer
consults the map for every :name: it finds, and a request per lookup would turn
each message into a cascade.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/web/src/utils/customEmoji.ts

@@ -0,0 +1,39 @@
+import type { SpaceEmoji } from '../api/client';
+
+/**
+ * Troca `:nome:` pela sintaxe de imagem do markdown, para o renderizador
+ * existente desenhar o emoji próprio sem precisar de plugin.
+ *
+ * Trechos de código são preservados: `:nome:` dentro de crase é texto que a
+ * pessoa quis mostrar literalmente, e virar imagem ali seria destruir o que ela
+ * escreveu.
+ */
+export function renderCustomEmojis(
+  content: string,
+  lookup: (name: string) => SpaceEmoji | undefined,
+  urlOf: (filename: string) => string,
+): string {
+  if (!content.includes(':')) return content;
+
+  // Divide preservando blocos cercados (```) e código em linha (`) —
+  // os delimitadores ficam nos pedaços ímpares e passam intactos.
+  const parts = content.split(/(```[\s\S]*?```|`[^`]*`)/g);
+
+  return parts
+    .map((part, i) => {
+      if (i % 2 === 1) return part;
+      return part.replace(/:([a-z0-9_]{2,32}):/gi, (whole, name: string) => {
+        const emoji = lookup(name.toLowerCase());
+        if (!emoji) return whole;
+        // O alt conserva `:nome:` para que copiar a mensagem devolva o texto
+        // original, e é por ele que o renderizador reconhece o emoji.
+        return `![:${emoji.name}:](${urlOf(emoji.filename)})`;
+      });
+    })
+    .join('');
+}
+
+/** O renderizador usa isto para dar tamanho de emoji em vez de imagem normal. */
+export function isCustomEmojiAlt(alt: string | undefined): boolean {
+  return !!alt && /^:[a-z0-9_]{2,32}:$/i.test(alt);
+}