refactor(processing): remove unused processing start field

processing_started_at 没有任何判据消费（终态只看尝试上限），删除字段、仓储接口与方法、主泵写入、重放清理、SELECT/映射与 H2 fixture，并新增 V7 DROP COLUMN；V1–V6 历史迁移不动。

验证：./gradlew test 123 tests / 0 fail；V7 的 DROP 与最终 schema 断言需真实 PG，本机无 Docker 时 FlywayMigrationTest 按既有 assumeTrue 跳过。

src/test/kotlin/com/gzzn/omms/msgexchange/infra/persistence/jdbc/FlywayMigrationTest.kt

@@ -48,7 +48,7 @@ class FlywayMigrationTest {
                     while (rs.next()) {
                         records.add(Triple(rs.getString("version"), rs.getString("script"), rs.getBoolean("success")))
                     }
-                    assertTrue(records.size >= 5, "flyway_schema_history must record all migrations")
+                    assertTrue(records.size >= 7, "flyway_schema_history must record all migrations")
                     assertEquals("1", records[0].first)
                     assertEquals("V1__flight_state_baseline.sql", records[0].second)
                     assertEquals("2", records[1].first)
@@ -59,6 +59,10 @@ class FlywayMigrationTest {
                     assertEquals("V4__backfill_closure.sql", records[3].second)
                     assertEquals("5", records[4].first)
                     assertEquals("V5__cutover_seed.sql", records[4].second)
+                    assertEquals("6", records[5].first)
+                    assertEquals("V6__enqueued_at.sql", records[5].second)
+                    assertEquals("7", records[6].first)
+                    assertEquals("V7__drop_processing_started_at.sql", records[6].second)
                     assertTrue(records.all { it.third })
                 }
 
@@ -88,7 +92,7 @@ class FlywayMigrationTest {
                     "SELECT column_name FROM information_schema.columns WHERE table_name = 'proc_state' " +
                         "AND column_name IN ('received_at', 'enqueued_at', 'backfill_at', 'backfill_next_at', " +
                         "'backfill_attempts', 'backfill_error', 'backfill_abandoned_at', " +
-                        "'backfill_abandoned_reason', 'processing_started_at')",
+                        "'backfill_abandoned_reason')",
                 ).use { rs ->
                     val cols = mutableSetOf<String>()
                     while (rs.next()) cols.add(rs.getString("column_name"))
@@ -96,12 +100,20 @@ class FlywayMigrationTest {
                         setOf(
                             "received_at", "enqueued_at", "backfill_at", "backfill_next_at", "backfill_attempts",
                             "backfill_error", "backfill_abandoned_at", "backfill_abandoned_reason",
-                            "processing_started_at",
                         ),
                         cols,
                     )
                 }
 
+                // V7：PROC_STATE 不再保留处理开始时间（无判据消费）
+                stmt.executeQuery(
+                    "SELECT count(*) FROM information_schema.columns WHERE table_name = 'proc_state' " +
+                        "AND column_name = 'processing_started_at'",
+                ).use { rs ->
+                    assertTrue(rs.next())
+                    assertEquals(0, rs.getInt(1), "V7 必须已删除 PROCESSING_STARTED_AT")
+                }
+
                 // V6：入队时间是超期判据 R 的比较对象，必须非空（received_at 则允许为 NULL）
                 stmt.executeQuery(
                     "SELECT is_nullable FROM information_schema.columns " +

src/test/kotlin/com/gzzn/omms/msgexchange/infra/persistence/jdbc/InboxLifecycleJdbcSqlTest.kt

@@ -142,15 +142,6 @@ class InboxLifecycleJdbcSqlTest {
         assertEquals(InboxCursorRepository.Cursor(42L, t0), cursor.load())
     }
 
-    @Test
-    fun `processing start is written once and is not refreshed`() {
-        seed(11L, t0)
-        proc.markProcessingStartedIfAbsent(11L, t0.plusSeconds(10))
-        proc.markProcessingStartedIfAbsent(11L, t0.plusSeconds(20))
-
-        assertEquals(t0.plusSeconds(10), proc.find(11L)!!.processingStartedAt)
-    }
-
     @Test
     fun `pipeline transaction rolls back terminal and outbox writes after a late failure`() {
         seed(11L, t0)
@@ -232,7 +223,6 @@ class InboxLifecycleJdbcSqlTest {
               backfill_error    VARCHAR(512),
               backfill_abandoned_at     TIMESTAMP WITH TIME ZONE,
               backfill_abandoned_reason VARCHAR(64),
-              processing_started_at TIMESTAMP WITH TIME ZONE,
               updated_at        TIMESTAMP WITH TIME ZONE NOT NULL,
               CONSTRAINT uk_proc_identity UNIQUE (identity_key)
             )

src/test/kotlin/com/gzzn/omms/msgexchange/infra/retry/ReplayServiceTest.kt

@@ -22,7 +22,6 @@ class ReplayServiceTest {
         fun seed(id: Long, status: ProcStatus, ec: ErrorClass?) {
             rows[id] = ProcState(
                 id, status, identityKey = "k$id", attempts = 3, errorClass = ec, lastError = "x",
-                processingStartedAt = Instant.parse("2026-09-08T03:00:00Z"),
             )
         }
 
@@ -34,7 +33,6 @@ class ReplayServiceTest {
         override fun find(msgId: Long): ProcState? = rows[msgId]
         override fun findSuccessTerminal(msgId: Long): Boolean = rows[msgId]?.state == ProcStatus.SUCCEEDED
         override fun headUnfinished(): ProcState? = null
-        override fun markProcessingStartedIfAbsent(msgId: Long, now: Instant) = Unit
         override fun tryBindIdentity(msgId: Long, identityKey: String): Boolean = true
         override fun ownerOfIdentity(identityKey: String): Long? = null
         override fun update(
@@ -69,7 +67,7 @@ class ReplayServiceTest {
                 val s = rows[id]!!
                 if (s.errorClass != null && s.errorClass in errorClasses &&
                     (s.state == ProcStatus.FAILED || s.state == ProcStatus.DEAD)) {
-                    rows[id] = s.copy(state = ProcStatus.PENDING, attempts = 0, nextAttemptAt = null, processingStartedAt = null)
+                    rows[id] = s.copy(state = ProcStatus.PENDING, attempts = 0, nextAttemptAt = null)
                     n++
                 }
             }
@@ -92,7 +90,6 @@ class ReplayServiceTest {
         assertEquals(ProcStatus.PENDING, repo.rows[1]!!.state)
         assertEquals(0, repo.rows[1]!!.attempts)
         assertNull(repo.rows[1]!!.nextAttemptAt)
-        assertNull(repo.rows[1]!!.processingStartedAt)
         assertEquals(ProcStatus.DEAD, repo.rows[2]!!.state)   // MALFORMED 永不被重放
         assertEquals(ProcStatus.PENDING, repo.rows[3]!!.state)
     }