iOS: fix phantom lastExecutedAt when BGTask expires before purchase

claimPlanForExecution no longer sets lastExecutedAt — only advances
nextExecutionAt to prevent double-purchase. lastExecutedAt is now set
exclusively by saveTransactionAndAdvance after a transaction is actually
persisted. This prevents the UI showing a "last executed" time with no
corresponding transaction when a BGTask is killed mid-execution.

Also saves a failed transaction when credentials are missing instead of
silently returning, so the user sees what went wrong.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: vitnehasil

accbot-ios/AccBot/Data/Local/Database/Dao/DcaPlanDao.swift

@@ -142,17 +142,19 @@ final class DcaPlanDao {
     /// Atomically claim a plan for execution by advancing nextExecutionAt,
     /// but only if it is still in the past (or null). Returns true if claimed,
     /// false if another task already claimed it.
+    /// Note: lastExecutedAt is NOT set here — it is only set after a transaction
+    /// is actually saved, so the UI never shows a "last executed" time without
+    /// a corresponding transaction.
     func claimPlanForExecution(id: Int64, now: Date, nextExecutionAt: Date) throws -> Bool {
         try dbPool.write { db in
             try db.execute(
                 sql: """
                     UPDATE dca_plans
-                    SET nextExecutionAt = ?, lastExecutedAt = ?
+                    SET nextExecutionAt = ?
                     WHERE id = ? AND (nextExecutionAt IS NULL OR nextExecutionAt <= ?)
                     """,
                 arguments: [
                     nextExecutionAt.timeIntervalSince1970,
-                    now.timeIntervalSince1970,
                     id,
                     now.timeIntervalSince1970,
                 ]

accbot-ios/AccBot/Service/DcaExecutionEngine.swift

@@ -123,6 +123,19 @@ final class DcaExecutionEngine {
         let isSandbox = userPreferences.isSandboxMode()
         guard let credentials = credentialsStore.get(for: plan.exchange, isSandbox: isSandbox) else {
             logger.error("No credentials for \(plan.exchange.displayName) (sandbox=\(isSandbox))")
+            let failedTx = Transaction(
+                planId: plan.id,
+                exchange: plan.exchange,
+                crypto: plan.crypto,
+                fiat: plan.fiat,
+                fiatAmount: plan.amount,
+                cryptoAmount: 0,
+                price: 0,
+                fee: 0,
+                status: .failed,
+                errorMessage: "No API credentials configured (sandbox=\(isSandbox))"
+            )
+            saveTransactionAndAdvance(failedTx, plan: plan, now: now)
             return
         }