refactor: companion FrameServer and related (substantive only, no Black)

Reapply refactor from ce8381a (replace monolithic FrameServer with thin
pymc_core subclass, re-export constants, SQLite persistence hooks) while
preserving pre-refactor whitespace where patch applied cleanly. Remaining
files match refactor commit exactly. Diff vs ce8381a is whitespace-only.

Co-authored-by: Cursor <cursoragent@cursor.com>

repeater/__init__.py

@@ -3,6 +3,7 @@ try:
 except ImportError:
     try:
         from importlib.metadata import version
+
         __version__ = version("pymc_repeater")
     except Exception:
         __version__ = "unknown"