feat(review): re-read an open Needs review page when its queue changes

An open Needs review page now follows the queue without polling. A new
server-sent event stream, GET /prompt_manager/review/events, sends
"ready" when it opens and "change" after every committed queue change:
an item the scan or the monitor recorded, a spelling refresh, a decision
from any tab, and a restore. database/review_changes.py notifies after
the commit from whichever thread wrote it. The page re-reads its first
page on each event, keeps the open item, its choice and an unanswered
decision, and re-reads the SAVED VIEWS count with it. When the open item
left the queue, the next one opens with a note. A keepalive comment
every 25 s lets the server drop a closed tab; shutdown ends every stream.

Leaving the page mid-decision now releases that decision's capture.

D-NEW-F1 is renumbered D119 in markers and TESTING.md.

py/api/__init__.py

@@ -9,6 +9,7 @@ Split into domain-specific mixins for maintainability:
 - JobRoutesMixin: the session job registry behind Activity (py/api/jobs.py)
 - ReviewRoutesMixin: the Needs review queue, item detail and decisions
 - ReviewRecordingMixin: Needs review recording for the scan and the monitor
+- ReviewEventsMixin: the Needs review change stream an open page re-reads on
 """
 
 import asyncio
@@ -31,6 +32,7 @@ from .vocabulary import VocabularyRoutesMixin
 from .annotations import AnnotationRoutesMixin
 from .review import ReviewRoutesMixin
 from .review_recording import ReviewRecordingMixin
+from .review_events import ReviewEventsMixin
 from .saved_views import SavedViewRoutesMixin
 from .history import HistoryRoutesMixin
 from .images import ImageRoutesMixin
@@ -260,6 +262,7 @@ class PromptManagerAPI(
     AnnotationRoutesMixin,
     ReviewRoutesMixin,
     ReviewRecordingMixin,
+    ReviewEventsMixin,
     SavedViewRoutesMixin,
     HistoryRoutesMixin,
     PromptRoutesMixin,
@@ -457,6 +460,7 @@ class PromptManagerAPI(
         self._register_vocabulary_routes(routes)
         self._register_annotation_routes(routes)
         self._register_review_routes(routes)
+        self._register_review_event_routes(routes)
         self._register_saved_view_routes(routes)
         self._register_history_routes(routes)
         self._register_prompt_routes(routes)

py/api/review_events.py

@@ -0,0 +1,130 @@
+"""Needs review change stream (#249 PR F2).
+
+``GET /prompt_manager/review/events`` is a server-sent event stream for an
+open Needs review page: ``event: ready`` when it opens, then
+``event: change`` after each committed queue change (an item the scan or the
+monitor recorded, a decision from any tab, a spelling refresh) and after a
+restore. Notices that arrive while one is being written collapse into one.
+The page re-reads its queue on each; nothing polls. The events carry no data
+(``{}``), so nothing about the queue is pushed.
+
+A ``: keepalive`` comment every ``HEARTBEAT_SECONDS`` lets the server notice
+a closed tab (its write fails) and drop the subscription; clients ignore
+comments. Shutdown (``close_review_events``, an ``on_shutdown`` hook) ends
+every stream, and a stream asked for afterwards is a 503.
+"""
+
+import asyncio
+
+from aiohttp import web
+
+try:
+    from ...database.execution_history import subscribe_history_restore
+    from ...database.review_changes import subscribe_review_changes
+except ImportError:
+    from database.execution_history import subscribe_history_restore
+    from database.review_changes import subscribe_review_changes
+
+EVENTS_URL = "/prompt_manager/review/events"
+HEARTBEAT_SECONDS = 25
+_READY = b"event: ready\ndata: {}\n\n"
+_CHANGE = b"event: change\ndata: {}\n\n"
+_KEEPALIVE = b": keepalive\n\n"
+
+
+class _Stream:
+    """One open stream: a change flag set from any thread, and a close flag."""
+
+    def __init__(self, loop: asyncio.AbstractEventLoop) -> None:
+        self._loop = loop
+        self.changed = asyncio.Event()
+        self.closed = asyncio.Event()
+
+    def wake(self) -> None:
+        """Thread-safe: mark a change (the loop may already be gone)."""
+        try:
+            self._loop.call_soon_threadsafe(self.changed.set)
+        except RuntimeError:  # the loop closed
+            pass
+
+    async def next(self, timeout: float) -> str:
+        """``change``, ``closed`` or ``idle`` (nothing within ``timeout``)."""
+        waits = {
+            asyncio.ensure_future(self.changed.wait()): "change",
+            asyncio.ensure_future(self.closed.wait()): "closed",
+        }
+        try:
+            done, _ = await asyncio.wait(
+                waits, timeout=timeout, return_when=asyncio.FIRST_COMPLETED
+            )
+        finally:
+            for waiter in waits:
+                waiter.cancel()
+        if self.closed.is_set():
+            return "closed"
+        if done:
+            self.changed.clear()
+            return "change"
+        return "idle"
+
+
+class ReviewEventsMixin:
+    """The Needs review change stream."""
+
+    def _register_review_event_routes(self, routes: web.RouteTableDef) -> None:
+        routes.get(EVENTS_URL)(self.review_events)
+
+    def _review_streams(self) -> set:
+        streams = getattr(self, "_open_review_streams", None)
+        if streams is None:
+            streams = self._open_review_streams = set()
+        return streams
+
+    def review_stream_count(self) -> int:
+        """How many change streams are open (tests and diagnostics)."""
+        return len(self._review_streams())
+
+    async def close_review_events(self, app) -> None:
+        """End every open stream; later requests get 503 (shutdown hook)."""
+        self._review_events_closed = True
+        for stream in list(self._review_streams()):
+            stream.closed.set()
+
+    async def review_events(self, request: web.Request) -> web.StreamResponse:
+        if getattr(self, "_review_events_closed", False):
+            return web.json_response(
+                {"success": False, "error": "The server is shutting down."},
+                status=503,
+            )
+        stream = _Stream(asyncio.get_running_loop())
+        db_path = self.db.model.db_path
+        unsubscribers = (
+            subscribe_review_changes(db_path, stream.wake),
+            subscribe_history_restore(db_path, stream.wake),
+        )
+        streams = self._review_streams()
+        streams.add(stream)
+        response = web.StreamResponse(
+            status=200,
+            headers={
+                "Content-Type": "text/event-stream",
+                "Cache-Control": "no-store",
+                "X-Accel-Buffering": "no",
+            },
+        )
+        try:
+            await response.prepare(request)
+            await response.write(_READY)
+            while True:
+                outcome = await stream.next(HEARTBEAT_SECONDS)
+                if outcome == "closed":
+                    break
+                await response.write(_CHANGE if outcome == "change" else _KEEPALIVE)
+            await response.write_eof()
+        except (ConnectionError, RuntimeError) as exc:  # the tab went away
+            self.logger.debug(f"Needs review stream closed ({exc})")
+        finally:
+            streams.discard(stream)
+            for unsubscribe in unsubscribers:
+                unsubscribe()
+        return response