Remove three kinds of duplication in the backend

Stage 2, items 1, 4, and 5 of `plan/17-refactor.md`.

**One path resolver in `worldstate`.** `apply_delta` and `apply_override` routed
`flags.<name>`, `milestones.<id>`, `world.<stat>`, `player.<stat>`, and
`npc.<id>.<stat>` with parallel code, about 100 lines each. `_resolve` now says
what a path points at and returns either a target or the rejection to report.
Each function keeps its own write rule, because the rules genuinely differ: an
override sets a number rather than adding to it, ignores `cooldown`,
`max_delta_per_turn`, and the rule that a counter only counts up, and can un-set
a milestone.

A differential check ran both implementations over 3960 payloads: twenty paths,
fourteen values, three starting states, plus every three-path combination. The
results are identical except that 674 rejections from `apply_override` now carry
a `fix` string. `apply_delta` already worded those, and the world-state editor
renders them, so an override that names an unknown flag now explains itself the
way a delta does.

**`sse`, `SSE_HEADERS`, and `turn_error` move to `app/sse.py`.** Two routers
stream, and `chat.py` had to import from `routers.adventures` to reach them.

**`get_adventure_or_404` becomes the `current_adventure` dependency.** All 32
handlers repeated the call as their first statement. The ownership check now
reads in the signature and runs before the body. FastAPI caches a dependency for
one request, so the handler's `db` is the session the adventure came from.

The generated OpenAPI document is byte-identical except on `rename_branch`,
where `branch_id` is now listed before `adventure_id`, because that handler no
longer names `adventure_id` itself. Parameter order in the document is
cosmetic.

Six tests in `test_state_revert.py` call `undo_turn` and `retry_action`
directly rather than over HTTP. They pass the adventure they already hold
instead of an id.

549 tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014Dix4oGV3njgWRdu7P9t6r

backend/app/routers/adventures/actions.py

@@ -11,18 +11,17 @@ from sqlalchemy.orm import Session
 from ... import models, schemas, tree
 from ...database import get_db
 
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 from .nodes import delete_turn
 from .paging import ACTION_PAGE, action_window, annotate_takes
 
 
 @router.get("/{adventure_id}/actions", response_model=schemas.ActionPage)
 def list_actions(
-    adventure_id: int,
     before_id: int | None = None,
     limit: int = ACTION_PAGE,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Returns a page of the story, working backwards from the newest action.
 
@@ -30,7 +29,6 @@ def list_actions(
     asks for what comes before it. Omit `before_id` for the newest window. See
     `action_window` for why this anchors on a row rather than an offset.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     limit = max(1, min(limit, ACTION_PAGE * 4))
     actions, total, has_more = action_window(
         db, adventure, before_id=before_id, limit=limit
@@ -51,9 +49,8 @@ def update_action(
     action_id: int,
     payload: schemas.ActionUpdate,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    get_adventure_or_404(adventure_id, db, user)
     action = db.get(models.Action, action_id)
     if action is None or action.adventure_id != adventure_id:
         raise HTTPException(404, "Action not found")
@@ -70,9 +67,8 @@ def delete_action(
     adventure_id: int,
     action_id: int,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    adventure = get_adventure_or_404(adventure_id, db, user)
     action = db.get(models.Action, action_id)
     if action is None or action.adventure_id != adventure_id:
         raise HTTPException(404, "Action not found")