Let a player's own turn be played again

Retry has always given an AI turn another take. Nothing gave one to the
message the player wrote, so the only way to change something you had typed
was to overwrite it -- destructively, losing whatever the story made of it.
That is the half of the tree the player asked for first, and it never existed:
`POST /actions/{id}/fork` answers 400 unless a second take is already there,
so "branch from here" was not a thing the API could do.

`tree.branch_at` is the missing piece. `fork` moves a node that already exists
onto a line of its own; this is the same branch with nothing in it yet, for a
take that has not been written. The head lands one depth short, so the next
node written is the new take -- same depth, same parent, resolved from the
path by `place_action` without being told. The line being left keeps its node,
keeps it live, and keeps everything played after it. Nothing is copied.

Two things the tests said rather than confirmed. A player turn is never the
tip: the reply to it is, so retaking even the newest thing typed still owes a
branch, and the guard against forking for nothing only fires for a player
action with no reply under it. And player text is stored formatted, so a test
that compares it whole is testing the formatter.

421 tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Dvvqn9ZDR4ixeFPHNbww7

backend/app/schemas.py

@@ -254,6 +254,12 @@ class ActionCreate(BaseModel):
     after_id: int | None = None
 
 
+class TakeCreate(BaseModel):
+    """Another take of a turn the player wrote themselves (SP9)."""
+
+    text: ActionText
+
+
 class AdventureOut(ORMModel):
     id: int
     scenario_id: int | None