Add Syndicate Invite Token System (Secure Links)

- Added route + handler: GenerateInviteLinkHandler to create signed tokens
- Added handler: JoinSyndicateWithTokenHandler to join using invite token
- Integrated secure token generation via helpers.GenerateSecureToken()
- Created DB model: syndicate_invite_tokens (assumed pre-existing)
- Updated syndicate view template to allow managers to generate links
- Flash messaging for invite success/failure
- Invite links are scoped to manager role and valid for 48 hours

storage/schema.go

@@ -209,3 +209,18 @@ CREATE TABLE IF NOT EXISTS syndicate_invites (
 	FOREIGN KEY(syndicate_id) REFERENCES syndicates(id),
 	FOREIGN KEY(invited_user_id) REFERENCES users(id)
 );`
+
+const SchemaSyndicateInviteTokens = `
+CREATE TABLE IF NOT EXISTS syndicate_invite_tokens (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  syndicate_id INTEGER NOT NULL,
+  token TEXT NOT NULL UNIQUE,
+  invited_by_user_id INTEGER NOT NULL,
+  accepted_by_user_id INTEGER,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+  accepted_at TIMESTAMP,
+  expires_at TIMESTAMP,
+  FOREIGN KEY (syndicate_id) REFERENCES syndicates(id),
+  FOREIGN KEY (invited_by_user_id) REFERENCES users(id),
+  FOREIGN KEY (accepted_by_user_id) REFERENCES users(id)
+);`