feat: flag accounts Reddit considers unestablished and re-test them

New accounts cannot start chats until Reddit considers them established.
Until now the bot would burn a candidate on every invitation Reddit was
always going to reject, with nothing in the dashboard explaining why the
account looked idle.

Two signals feed the verdict. A periodic profile check reads the
account's own age, karma and email verification from about.json and
compares them against configurable thresholds — that is what flags a
brand new account before it wastes anything. A rejected invitation is
authoritative: Reddit has told us directly, so the account is held out of
sending with a backoff that doubles per rejection, and a delivered
invitation clears the flag outright.

An unestablished account takes no new posts (canAcceptPost), raises an
attention item, reports an `unestablished` sending state, and shows its
age/karma evidence on the Accounts page. An unchecked account is always
allowed through, so a first run is never blocked.

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

src/database/client.ts

@@ -456,6 +456,27 @@ export function runMigrations(db: Database.Database): void {
       CREATE INDEX IF NOT EXISTS idx_reddit_recipient_account_status_due
         ON reddit_recipient_account_status(checked_at);
     `),
+    () => db.exec(`
+      CREATE TABLE IF NOT EXISTS reddit_account_establishment (
+        account_id TEXT PRIMARY KEY,
+        state TEXT NOT NULL CHECK(state IN ('unknown', 'established', 'unestablished')),
+        reasons_json TEXT NOT NULL DEFAULT '[]',
+        account_created_at INTEGER,
+        link_karma INTEGER,
+        comment_karma INTEGER,
+        total_karma INTEGER,
+        has_verified_email INTEGER,
+        profile_checked_at INTEGER,
+        last_probe_at INTEGER,
+        last_probe_failure TEXT,
+        probe_failure_count INTEGER NOT NULL DEFAULT 0,
+        retry_at INTEGER,
+        updated_at INTEGER NOT NULL,
+        FOREIGN KEY (account_id) REFERENCES reddit_accounts(account_id) ON DELETE CASCADE
+      );
+      CREATE INDEX IF NOT EXISTS idx_reddit_account_establishment_state
+        ON reddit_account_establishment(state, retry_at);
+    `),
   ];
 
   let currentVersion = db.pragma('user_version', { simple: true }) as number;