chore: implement database crud for AI seat usage (#22681)

Creates a new table `ai_seat_state` to keep track of when users consume an ai_seat. Once a user consumes an AI seat, they will forever in this table (as it stands today).

coderd/database/migrations/000439_ai_seat_state.down.sql

@@ -0,0 +1,3 @@
+DROP TABLE ai_seat_state;
+
+DROP TYPE ai_seat_usage_reason;

coderd/database/migrations/000439_ai_seat_state.up.sql

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+CREATE TYPE ai_seat_usage_reason AS ENUM (
+	'aibridge',
+	'task'
+);
+
+CREATE TABLE ai_seat_state (
+	user_id uuid NOT NULL PRIMARY KEY REFERENCES users (id) ON DELETE CASCADE,
+	first_used_at timestamptz NOT NULL,
+	last_used_at timestamptz NOT NULL,
+	last_event_type ai_seat_usage_reason NOT NULL,
+	last_event_description text NOT NULL,
+	updated_at timestamptz NOT NULL
+);

coderd/database/migrations/testdata/fixtures/000439_ai_seat_state.up.sql

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+INSERT INTO
+	ai_seat_state (
+		user_id,
+		first_used_at,
+		last_used_at,
+		last_event_type,
+		last_event_description,
+		updated_at
+	)
+VALUES
+	('30095c71-380b-457a-8995-97b8ee6e5307', NOW(), NOW(), 'task'::ai_seat_usage_reason, 'Used for AI task', NOW());