build: pull quart-session from gitea registry, bump to python 3.14

- Drop the quart-session git submodule and its uv workspace member entry
- Resolve quart-session from the private Gitea PyPI index instead
- Pin the new index as explicit so only quart-session is fetched from it
- Require quart-session >= 3.0.2, the version published in the registry
- Raise requires-python to >=3.14 and set .python-version to 3.14
- Switch the image base to ghcr.io/astral-sh/uv:python3.14-trixie-slim
- Relax runtime pins from == to >= so uv.lock stays the single source
- Relock all 50 packages for CPython 3.14, dropping 3.13-only markers
- Delete the stale hand-maintained requirements.txt, unused by the build

Verified with uv lock --check, uv sync --frozen and a docker build; the
container reports CPython 3.14.7 and imports quart_session and convex.
Test suite is unchanged at 131 passed, 4 pre-existing wide_event failures.

.python-version

@@ -1 +1 @@
-3.13
+3.14