feat: add database schema, migrations, and API endpoints for span annotations

- Add span_label_types and span_annotations tables to schema
- Seed default span label types (bull_flag, bear_flag, etc.)
- Implement CRUD API endpoints for span label types
- Implement CRUD API endpoints for span annotations
- Add time swap validation in POST endpoint (start_time <= end_time)

drizzle/0003_demonic_captain_flint.sql

@@ -0,0 +1,27 @@
+CREATE TABLE `span_annotations` (
+	`id` integer PRIMARY KEY AUTOINCREMENT NOT NULL,
+	`chart_id` integer NOT NULL,
+	`start_time` integer NOT NULL,
+	`end_time` integer NOT NULL,
+	`label` text NOT NULL,
+	`confidence` integer,
+	`outcome` text,
+	`notes` text,
+	`sub_spans` text,
+	`color` text DEFAULT '#2196F3' NOT NULL,
+	`created_at` integer NOT NULL,
+	FOREIGN KEY (`chart_id`) REFERENCES `charts`(`id`) ON UPDATE no action ON DELETE no action
+);
+--> statement-breakpoint
+CREATE TABLE `span_label_types` (
+	`id` integer PRIMARY KEY AUTOINCREMENT NOT NULL,
+	`name` text NOT NULL,
+	`display_name` text NOT NULL,
+	`color` text NOT NULL,
+	`hotkey` text,
+	`is_active` integer DEFAULT 1 NOT NULL,
+	`sort_order` integer DEFAULT 0 NOT NULL,
+	`created_at` integer NOT NULL
+);
+--> statement-breakpoint
+CREATE UNIQUE INDEX `span_label_types_name_unique` ON `span_label_types` (`name`);