Change ordering_key column from VARCHAR(512) to TEXT

ordering_key is an arbitrary string (often a URL in Fedify), so the
previous VARCHAR(512) limit could silently truncate or reject values
longer than 512 characters at insert time with an opaque DB error.

Switch the column to TEXT so there is no fixed-length cap.  Because
InnoDB requires a prefix length when indexing TEXT columns, the
composite index now uses ordering_key(766):

  766 chars * 4 bytes (utf8mb4) + 8 bytes (DATETIME(6)) = 3072 bytes

which exactly hits InnoDB's 3072-byte key limit.

Existing tables created with the old VARCHAR(512) definition will
continue to work; operators who want to lift the 512-char cap must
ALTER the column manually (or drop and recreate the table via drop()).

#599 (comment)

Co-Authored-By: claude-sonnet-4-6 <claude-sonnet-4-6@anthropic.com>

Author: dahlia

packages/mysql/src/mq.ts

@@ -539,10 +539,10 @@ export class MysqlMessageQueue implements MessageQueue {
       try {
         await this.#pool.query(
           `CREATE TABLE IF NOT EXISTS \`${this.#tableName}\` (
-            \`id\`            CHAR(36)     NOT NULL,
-            \`message\`       JSON         NOT NULL,
-            \`deliver_after\` DATETIME(6)  NOT NULL DEFAULT NOW(6),
-            \`ordering_key\`  VARCHAR(512) NULL     DEFAULT NULL,
+            \`id\`            CHAR(36)    NOT NULL,
+            \`message\`       JSON        NOT NULL,
+            \`deliver_after\` DATETIME(6) NOT NULL DEFAULT NOW(6),
+            \`ordering_key\`  TEXT        NULL     DEFAULT NULL,
             PRIMARY KEY (\`id\`)
           ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin`,
         );
@@ -558,9 +558,14 @@ export class MysqlMessageQueue implements MessageQueue {
         try {
           // Composite index to speed up #findOrderingKeyCandidate():
           // scans for the oldest ready message per ordering key.
+          // ordering_key uses a 766-character prefix because TEXT columns
+          // require a prefix length for indexing, and InnoDB's 3072-byte key
+          // limit in utf8mb4 (4 bytes/char) leaves room for 766 chars once the
+          // DATETIME(6) companion column (8 bytes) is included:
+          // 766 * 4 + 8 = 3072 bytes exactly.
           await this.#pool.query(
             `CREATE INDEX \`idx_${this.#tableName}_ok_da\`
-             ON \`${this.#tableName}\` (\`ordering_key\`, \`deliver_after\`)`,
+             ON \`${this.#tableName}\` (\`ordering_key\`(766), \`deliver_after\`)`,
           );
         } catch (e) {
           // Ignore duplicate index (ER_DUP_KEYNAME) from concurrent init