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/memory.py

@@ -27,6 +27,12 @@ CREATE TABLE IF NOT EXISTS exchanges (
     created_at TEXT NOT NULL
 );
 CREATE INDEX IF NOT EXISTS idx_session_created ON exchanges(session_id, created_at);
+
+CREATE TABLE IF NOT EXISTS sessions (
+    id TEXT PRIMARY KEY,
+    name TEXT,
+    created_at TEXT NOT NULL
+);
 """
 
 _conn: sqlite3.Connection | None = None
@@ -41,7 +47,10 @@ def _connection() -> sqlite3.Connection:
         if _conn is not None:
             _conn.close()
         cfg.db_path.parent.mkdir(parents=True, exist_ok=True)
-        _conn = sqlite3.connect(cfg.db_path)
+        # check_same_thread=False: the web server runs blocking work in a thread
+        # pool, so the singleton connection is touched from threads other than
+        # the one that created it. Safe here under single-user, low-concurrency use.
+        _conn = sqlite3.connect(cfg.db_path, check_same_thread=False)
         _conn.row_factory = sqlite3.Row
         _conn.executescript(SCHEMA)
         _conn_path = cfg.db_path
@@ -100,6 +109,70 @@ def recent(session_id: str, n: int = 10) -> list[Exchange]:
     ]
 
 
+def ensure_session(session_id: str, name: str | None = None) -> None:
+    """Create the session row if absent; set its name if one is given."""
+    now = datetime.now(timezone.utc).isoformat()
+    conn = _connection()
+    with conn:
+        conn.execute(
+            "INSERT INTO sessions (id, name, created_at) VALUES (?, ?, ?) "
+            "ON CONFLICT(id) DO NOTHING",
+            (session_id, name, now),
+        )
+        if name is not None:
+            conn.execute("UPDATE sessions SET name = ? WHERE id = ?", (name, session_id))
+
+
+def list_sessions() -> list[dict]:
+    """All known sessions (named rows + any session that has exchanges), newest first."""
+    conn = _connection()
+    rows = conn.execute(
+        """
+        SELECT s.id AS id,
+               s.name AS name,
+               COALESCE(s.created_at, MIN(e.created_at)) AS created_at
+        FROM sessions s
+        LEFT JOIN exchanges e ON e.session_id = s.id
+        GROUP BY s.id
+        UNION
+        SELECT e.session_id AS id, NULL AS name, MIN(e.created_at) AS created_at
+        FROM exchanges e
+        WHERE e.session_id NOT IN (SELECT id FROM sessions)
+        GROUP BY e.session_id
+        ORDER BY created_at DESC
+        """
+    ).fetchall()
+    return [{"id": r["id"], "name": r["name"]} for r in rows]
+
+
+def history(session_id: str) -> list[Exchange]:
+    """Full conversation for a session, oldest first."""
+    conn = _connection()
+    rows = conn.execute(
+        "SELECT id, session_id, role, content, created_at FROM exchanges "
+        "WHERE session_id = ? ORDER BY id ASC",
+        (session_id,),
+    ).fetchall()
+    return [
+        Exchange(
+            id=r["id"],
+            session_id=r["session_id"],
+            role=r["role"],
+            content=r["content"],
+            created_at=r["created_at"],
+        )
+        for r in rows
+    ]
+
+
+def delete_session(session_id: str) -> None:
+    """Remove a session and all its exchanges."""
+    conn = _connection()
+    with conn:
+        conn.execute("DELETE FROM exchanges WHERE session_id = ?", (session_id,))
+        conn.execute("DELETE FROM sessions WHERE id = ?", (session_id,))
+
+
 def recall(query: str, k: int = 5, session_id: str | None = None) -> list[Exchange]:
     """Top-k exchanges semantically similar to `query`, optionally scoped to a session."""
     [q_vec] = llm.embed([query])