feat: adopt Python 3.15 and harden caching and share links

- Switch local Python and the Docker base image to Python 3.15.
- Refresh locked dependencies with Python-version-specific resolution.
- Replace Pickle caching with native Redis strings, lists, and typed JSON.
- Validate cache TTLs and apply expiry atomically on Redis writes.
- Treat legacy binary cache values as misses and clear keys with SCAN.
- Ignore trailing prose after decoded spaces in share-link paths.
- Reject empty or slash-containing share IDs before database lookup.
- Add regression tests for plain and encoded spaces in share links.
- Update helper submodule for config lookup and tabular storage fixes.
- Update quart_common with JSON error response detection.
- Verify all 150 tests pass on CPython 3.15.0.

Dockerfile

@@ -1,4 +1,4 @@
-FROM ghcr.io/astral-sh/uv:python3.14-trixie-slim
+FROM ghcr.io/astral-sh/uv:python3.15-trixie-slim
 
 # Install app
 COPY . /app