feat: record stream end mode in telemetry

llm_calls gains a nullable stream_end TEXT column (auto backfill,
manual-mode trimming as in 1.3.8). TelemetryRecorder.record_llm_call
takes a new required keyword stream_end; the emitter reads it from the
response on attempt rows and writes NULL for failed, cache-hit and
terminal rows.

Red: 18 tests in tests/unit/test_telemetry.py and test_ports.py failed
against the pre-change source (missing column, 38-field signature,
stream_end absent from emitted rows).

src/polygateway/ports.py

@@ -290,7 +290,7 @@ class TelemetryStatusProvider(Protocol):
 
 @runtime_checkable
 class TelemetryRecorder(Protocol):
-    """遥测后端;38 字段冻结(M1 设计 §4.4 + issue #3/#4/#11/#16/#20 + 1.3.5/1.3.8),唯一调用点是 TelemetryEmitter。
+    """遥测后端;39 字段冻结(M1 设计 §4.4 + issue #3/#4/#11/#16/#20 + 1.3.5/1.3.8/1.3.10),唯一调用点是 TelemetryEmitter。
 
     新增参数不设默认值: 库外无第三方实现者(三项目迁移时删除了各自的同名
     Protocol),完整签名的成本为零,而少写一列会被 emitter 的降级吞成 warning。
@@ -357,4 +357,5 @@ class TelemetryRecorder(Protocol):
         total_latency_ms: int | None,
         stall_ms: int | None,
         stall_count: int | None,
+        stream_end: str | None,
     ) -> None: ...