feat: backfill_embeddings --re-embed-all flag for model swaps (T112.4)

Adds two new flags to the backfill script:

* --re-embed-all walks **every** memory (not just those without
  an existing embeddings row) and re-emits embedding_indexed
  events. The projector is INSERT OR REPLACE, so re-emitting an event
  for an existing memory replaces the prior vector. Use this when
  swapping embedding models — the default mode still keeps the Phase
  4 gap-fill behavior.
* --model M overrides Settings.embedding_model for this run.

The script also gains a small _build_client helper that returns
None for the pseudo path (no client needed) and a FeatherlessClient
otherwise; tests monkeypatch this to inject a Mock with canned
embeddings.

Adds tests/test_backfill_embeddings.py with three integration
tests: re-embed-all walks every memory, default mode skips existing
rows, and --model overrides the configured model end-to-end.

scripts/backfill_embeddings.py

@@ -8,8 +8,21 @@ Phase 4 ships the deterministic local pseudo-embedding so this script
 runs synchronously without a network round-trip — the LLMClient argument
 is not needed on the pseudo path. Phase 4.5+ will need a real client.
 
+T112 (Phase 4.5) adds two flags:
+
+* ``--re-embed-all`` walks **every** memory regardless of whether it
+  already has an ``embeddings`` row. Useful when swapping embedding
+  models — the projector is INSERT OR REPLACE, so re-emitting an event
+  for an existing memory replaces the prior vector. Without this flag,
+  the script keeps the Phase 4 behavior of only filling in gaps.
+* ``--model M`` overrides ``Settings.embedding_model`` for this run.
+  Defaults to the configured model (which itself defaults to
+  ``"pseudo-sha256-384"``).
+
 Run from the repo root:
     .venv/bin/python scripts/backfill_embeddings.py [--limit N] [--dry-run]
+    .venv/bin/python scripts/backfill_embeddings.py --re-embed-all
+    .venv/bin/python scripts/backfill_embeddings.py --re-embed-all --model bge-small-en-v1.5
 """
 
 from __future__ import annotations
@@ -17,11 +30,12 @@ from __future__ import annotations
 import argparse
 import asyncio
 
-from chat.config import load_settings
+from chat.config import Settings, load_settings
 from chat.db.connection import open_db
 from chat.db.migrate import apply_migrations
 from chat.eventlog.log import append_and_apply
 from chat.services.embeddings import (
+    DEFAULT_EMBEDDING_MODEL,
     FALLBACK_EMBEDDING_MODEL,
     generate_embedding,
 )
@@ -34,6 +48,24 @@ import chat.state.memory  # noqa: F401
 import chat.state.world  # noqa: F401
 
 
+def _build_client(settings: Settings):
+    """Construct an LLMClient for the backfill run.
+
+    Default-model runs (the pseudo path) don't need a client, so we
+    return ``None`` and ``generate_embedding`` skips the call. Non-default
+    models route through the real client; injectable via monkeypatch in
+    tests.
+    """
+    if settings.embedding_model == DEFAULT_EMBEDDING_MODEL:
+        return None
+    from chat.llm.featherless import FeatherlessClient
+
+    return FeatherlessClient(
+        api_key=settings.featherless_api_key,
+        base_url=settings.featherless_base_url,
+    )
+
+
 async def main() -> None:
     parser = argparse.ArgumentParser(description=__doc__)
     parser.add_argument(
@@ -47,23 +79,51 @@ async def main() -> None:
         action="store_true",
         help="Print the count of memories needing embeddings, then exit.",
     )
+    parser.add_argument(
+        "--re-embed-all",
+        action="store_true",
+        help=(
+            "Walk every memory (not just those without an embeddings row) "
+            "and re-emit embedding_indexed events. Use this when swapping "
+            "embedding models so the existing rows get replaced."
+        ),
+    )
+    parser.add_argument(
+        "--model",
+        type=str,
+        default=None,
+        help=(
+            "Embedding model identifier. Overrides Settings.embedding_model "
+            "for this run; default uses the configured model."
+        ),
+    )
     args = parser.parse_args()
 
     settings = load_settings()
     settings.db_path.parent.mkdir(parents=True, exist_ok=True)
     apply_migrations(settings.db_path)
 
+    model = args.model or settings.embedding_model
+    # Override the settings instance so ``_build_client`` sees the
+    # effective model when deciding whether to construct a real client.
+    settings = settings.model_copy(update={"embedding_model": model})
+    client = _build_client(settings)
+
     with open_db(settings.db_path) as conn:
-        sql = (
-            "SELECT m.id, m.pov_summary FROM memories m "
-            "LEFT JOIN embeddings e ON e.memory_id = m.id "
-            "WHERE e.memory_id IS NULL "
-            "ORDER BY m.id"
-        )
+        if args.re_embed_all:
+            sql = "SELECT m.id, m.pov_summary FROM memories m ORDER BY m.id"
+        else:
+            sql = (
+                "SELECT m.id, m.pov_summary FROM memories m "
+                "LEFT JOIN embeddings e ON e.memory_id = m.id "
+                "WHERE e.memory_id IS NULL "
+                "ORDER BY m.id"
+            )
         if args.limit is not None:
             sql += f" LIMIT {int(args.limit)}"
         rows = conn.execute(sql).fetchall()
-        print(f"Found {len(rows)} memories needing embeddings.")
+        mode = "re-embedding" if args.re_embed_all else "needing embeddings"
+        print(f"Found {len(rows)} memories {mode} (model={model}).")
         if args.dry_run:
             return
 
@@ -71,11 +131,12 @@ async def main() -> None:
         skipped = 0
         for memory_id, text in rows:
             result = await generate_embedding(
-                client=None,  # pseudo path: no client needed
+                client=client,
                 text=text or "",
+                model=model,
             )
             if result.model == FALLBACK_EMBEDDING_MODEL:
-                print(f"  Skipping memory_id={memory_id} (empty text)")
+                print(f"  Skipping memory_id={memory_id} (empty text or fallback)")
                 skipped += 1
                 continue
             append_and_apply(