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/deps.py

@@ -9,6 +9,7 @@ from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.orm import Session
 
 from ... import auth, models
+from ...database import get_db
 
 
 router = APIRouter(prefix="/api/adventures", tags=["adventures"])
@@ -23,3 +24,19 @@ def get_adventure_or_404(
     if adventure is None or adventure.user_id != user.id:
         raise HTTPException(404, "Adventure not found")
     return adventure
+
+
+def current_adventure(
+    adventure_id: int,
+    db: Session = Depends(get_db),
+    user: models.User = Depends(auth.get_current_user),
+) -> models.Adventure:
+    """Resolves the `{adventure_id}` in the path to the caller's own adventure.
+
+    Declare this in a handler's signature. The ownership check then reads in the
+    signature, where you look for it, and it runs before the handler body.
+
+    FastAPI caches a dependency for the length of one request, so the handler's
+    own `db` is the same session this one loaded the adventure from.
+    """
+    return get_adventure_or_404(adventure_id, db, user)