feat: persona chat loop, web UI, and local (Ollama) embeddings

Phase 1 — persona + persistent memory chat loop:
- lyra/persona.py + personas/lyra.md: editable identity/voice (friend-first,
  honest, never invents poker math)
- lyra/chat.py: turn loop assembling persona + cross-session recall + recent
  context, persisting both sides to SQLite
- lyra/session.py, lyra/__main__.py: session lifecycle + `lyra` REPL

Phase 1.25 — reuse the old web UI:
- vendored the prior single-page UI into lyra/web/static, repointed to
  same-origin
- lyra/web/server.py (FastAPI): serves the UI and backs its endpoint contract
  (/v1/chat/completions, session CRUD, health, inert thinking-stream) with the
  new chat loop + memory; SQLite stays the single source of truth
- `lyra-web` console script

Local backends — test for free, no OpenAI key:
- llm.embed routes via EMBED_BACKEND (cloud=OpenAI, local=Ollama /api/embed)
- simplified UI backend selector to Local (Ollama) / Cloud (OpenAI), default local
- memory connection opened check_same_thread=False for the threaded server

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/llm.py

@@ -36,7 +36,22 @@ def complete(messages: list[Message], backend: Backend = "local") -> str:
 
 
 def embed(texts: list[str]) -> list[list[float]]:
+    """Embed texts using the configured backend (EMBED_BACKEND: "cloud" or "local").
+
+    Note: OpenAI and Ollama embeddings live in different vector spaces (and
+    dimensions). A given database is tied to whichever backend created it — don't
+    switch EMBED_BACKEND against an existing DB or cosine recall will break.
+    """
     cfg = load()
+    if cfg.embed_backend == "local":
+        resp = httpx.post(
+            f"{cfg.local_base_url}/api/embed",
+            json={"model": cfg.local_embed_model, "input": texts},
+            timeout=120,
+        )
+        resp.raise_for_status()
+        return resp.json()["embeddings"]
+
     if not cfg.openai_api_key:
         raise RuntimeError("OPENAI_API_KEY is not set")
     client = OpenAI(api_key=cfg.openai_api_key)