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/server/drizzle/0017_purple_wildside.sql

@@ -0,0 +1,26 @@
+CREATE TABLE `space_emojis` (
+	`id` text PRIMARY KEY NOT NULL,
+	`space_id` text NOT NULL,
+	`name` text NOT NULL,
+	`filename` text NOT NULL,
+	`uploader_id` text,
+	`created_at` integer NOT NULL,
+	FOREIGN KEY (`space_id`) REFERENCES `spaces`(`id`) ON UPDATE no action ON DELETE cascade,
+	FOREIGN KEY (`uploader_id`) REFERENCES `users`(`id`) ON UPDATE no action ON DELETE set null
+);
+--> statement-breakpoint
+CREATE TABLE `space_stickers` (
+	`id` text PRIMARY KEY NOT NULL,
+	`space_id` text NOT NULL,
+	`name` text NOT NULL,
+	`filename` text NOT NULL,
+	`uploader_id` text,
+	`created_at` integer NOT NULL,
+	FOREIGN KEY (`space_id`) REFERENCES `spaces`(`id`) ON UPDATE no action ON DELETE cascade,
+	FOREIGN KEY (`uploader_id`) REFERENCES `users`(`id`) ON UPDATE no action ON DELETE set null
+);
+--> statement-breakpoint
+ALTER TABLE `messages` ADD `sticker_id` text;--> statement-breakpoint
+CREATE INDEX `idx_space_emojis_space` ON `space_emojis` (`space_id`);--> statement-breakpoint
+CREATE UNIQUE INDEX `idx_space_emojis_space_name` ON `space_emojis` (`space_id`,`name`);--> statement-breakpoint
+CREATE INDEX `idx_space_stickers_space` ON `space_stickers` (`space_id`);