fix(history): give each thread its own SQLite connection (fixes #39)

[calvinlow18]

Fixes #39.

The bug

SyncHistory caches a single sqlite3.Connection in self._conn, created lazily by whichever thread calls _get_conn() first. The daemon runs every sync through asyncio.to_thread(), so the next source lands on a different pool thread — and sqlite3 (with its default check_same_thread=True) refuses it:

SQLite objects created in a thread can only be used in that same thread.

The first source to finish wins the connection and succeeds; every other source raises. That matches #39's report exactly (4 of 5 sources failing), and it is connector-agnostic — #39 hit it with Google Drive, I hit it with outline: on 4 sources.

Why it's worse than it looks

SyncHistory.log() is called after run_sync() has already uploaded, and _run_entry_locked() catches the exception in its broad except Exception — which overwrites the just-set success state:

_scheduler_state[source] = { "status": status, "files_added": result.added, ... }  # success
...
await asyncio.to_thread(_history.log, ...)   # <-- raises here
except Exception as e:
    _scheduler_state[source] = {"status": "error", "error": str(e)}   # success is discarded

So the documents do reach the knowledge base, while /health, the dashboard and the history all report error for a sync that actually worked. Anything monitoring the daemon is permanently red, and the counts (files_added, etc.) are lost.

The fix

Hold the connection in a threading.local() so each thread gets its own. Also open with WAL and a busy timeout, so the now-genuinely-concurrent writers wait on the brief write lock instead of raising database is locked.

One file, no API change.

Reproduction

Against main, driving SyncHistory the way the daemon does:

h = SyncHistory(db_path=db)
async def log(i):
    await asyncio.to_thread(h.log, source=f"src{i}", kb_id=f"kb{i}",
                            status="success", started_at=0.0, files_added=1)
await asyncio.gather(*(log(i) for i in range(5)), return_exceptions=True)

Before: 4 of 5 raise ProgrammingError: SQLite objects created in a thread..., and the follow-up query() raises too.
After: sources logged OK: 5/5, rows in history: 5.

Also re-checked the single-threaded CLI path — log, query (plain / errors_only / kb_id), last_sync, clear, and a double close() — all unchanged.

[tjbck]

Should be addressed!