Give every action a branch and a depth

Phase 14 SP1. The tree goes into the schema and nothing reads it yet: a
`branches` table, `branch_id`/`depth` on actions and memories, a head pointer
on adventures, migrations 46-52, and a server-side backfill that re-reads every
existing adventure as a tree with one branch. `depth` holds the number `index`
already held, gaps included, so no story changes — a linear story *is* a tree
with one branch, which is what makes the SP0 baseline passing unmodified the
pass condition rather than a hope.

The writer had to come with it. No migration will ever visit a row written
after it ran, so columns backfilled today and populated next subphase would
leave a hole exactly the width of one deploy, and from SP2 on a row without a
branch is a row no read can see. `app/tree.py` owns that: one module, because a
node written without a branch fails by disappearing rather than by raising.

Three things the schema itself insisted on:

- `adventures.head_branch_id` is a plain integer, not a foreign key. Pointing
  both ways makes the two tables a cycle create_all cannot order, and its
  escape hatch needs an ALTER SQLite does not have. It is a cache, and a head
  naming a branch that is gone recovers onto the root.
- `lineage` is NOT NULL, so the backfill inserts `'[]'` and fills it in a
  second pass guarded on `json_array_length(lineage) = 0` — not `= '[]'`,
  because Postgres `json` has no equality operator.
- SQLite will not drop a column a foreign key names, which is how two existing
  tests broke: they simulated an old database by rewinding the stamp while
  leaving the new columns in place. Every ADD COLUMN migration is now
  idempotent, and `tests/test_tree_migration.py` builds a genuine schema 45 by
  rebuilding three tables from frozen DDL so the real ALTERs run.

297 tests green, 14 of them new. `branches` costs 0.1 kB of a 733.5 kB turn;
page load and index are byte-identical to the recorded figures.

The deploy that ships this needs one `VACUUM FULL actions;` on the direct
endpoint afterwards — it rewrites every row.

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

backend/tools/stress_session.py

@@ -137,7 +137,7 @@ from fastapi import Depends
 from fastapi.testclient import TestClient
 from sqlalchemy import text
 
-from app import auth, limits, memorybank, models, security, seed, worldstate
+from app import auth, limits, memorybank, models, security, seed, tree, worldstate
 from app.database import Base, SessionLocal, engine, get_db
 from app.main import app
 from app.providers import PromptParts
@@ -379,12 +379,14 @@ def add_second_adventure(db, rng: random.Random, user) -> int:
     db.add(other)
     db.flush()
     for i in range(6):
-        db.add(models.Action(
+        action = models.Action(
             adventure_id=other.id, index=i,
             type="ai" if i % 2 else "do",
             text=f"[second adventure] turn {i}. {prose(rng, 200)}",
             state_before=rich_script_state(i),
-        ))
+        )
+        tree.place_action(db, other, action)
+        db.add(action)
     memory = models.Memory(
         adventure_id=other.id,
         text="This memory belongs to the second adventure and must never be "
@@ -392,6 +394,7 @@ def add_second_adventure(db, rng: random.Random, user) -> int:
         source_start=0, source_end=5,
     )
     memorybank.set_vector(memory, [rng.uniform(-1.0, 1.0) for _ in range(EMBEDDING_DIMS)])
+    tree.place_memory(db, other, memory)
     db.add(memory)
     return other.id
 
@@ -483,7 +486,7 @@ def build_fixture(args, rng: random.Random) -> tuple[int, int]:
                     if retried else []
                 ), 0
                 body = NARRATION if is_ai else PLAYER_INPUT
-            db.add(models.Action(
+            action = models.Action(
                 adventure_id=adventure.id,
                 index=i,
                 type="ai" if is_ai else "do",
@@ -503,7 +506,13 @@ def build_fixture(args, rng: random.Random) -> tuple[int, int]:
                 world_state_before=(
                     rich_world_state(schema, i) if args.rich else None
                 ),
-            ))
+            )
+            # A fresh database is built by create_all and stamped LATEST, so no
+            # migration ever runs against it and the tree backfill never sees
+            # it. The fixture has to stamp its own nodes, or it would be the one
+            # database in the project whose actions have no branch.
+            tree.place_action(db, adventure, action)
+            db.add(action)
 
         for i in range(args.memories):
             memory = models.Memory(
@@ -522,6 +531,7 @@ def build_fixture(args, rng: random.Random) -> tuple[int, int]:
             memorybank.set_vector(
                 memory, [rng.uniform(-1.0, 1.0) for _ in range(EMBEDDING_DIMS)]
             )
+            tree.place_memory(db, adventure, memory)
             db.add(memory)
 
         if args.rich: