Remove snowflake from dependencies, documentation and code.

[ehclark]

Remove Snowflake storage backend

Summary

Removes the Snowflake object-store backend from AnyVar — including the
backend implementation, the snowflake-sqlalchemy dependency, all
Snowflake-specific ORM hooks, tests, and documentation.

The Postgres, DuckDB, and stateless (NoObjectStore) backends are
untouched.

Motivation

The Snowflake backend is no longer needed. Removing it eliminates a
heavy SQLAlchemy dialect dependency, simplifies the ORM layer (which had
dialect-specific with_variant(...) calls and a conditional
__table_args__), and reduces the surface area of the storage
abstraction.

Breaking changes

• ANYVAR_STORAGE_URI="snowflake://..." is no longer supported.
  create_storage() now raises the same generic
  ValueError("URI scheme snowflake is not implemented") it raises for
  any other unrecognized scheme.
• The snowflake-sqlalchemy dependency is removed from pyproject.toml.
• The pytest.mark.snowflake marker is no longer registered.

Changes

Deleted files

• src/anyvar/storage/snowflake.py — SnowflakeObjectStore,
  privatelink dialect monkey-patch, @compiles(Insert, "snowflake")
  PARSE_JSON helper.
• tests/unit/storage/test_snowflake.py,
  tests/unit/storage/test_snowflake_dyntable.py.
• docs/source/configuration/snowflake.rst.
• docs/source/api/api/storage/anyvar.storage.snowflake.rst
  (autosummary stub).

Modified files

• pyproject.toml — dropped snowflake-sqlalchemy~=1.8.2 and the
  snowflake: pytest marker.
• src/anyvar/anyvar.py — removed the "snowflake" branch in
  create_storage() and the matching docstring entry.
• src/anyvar/storage/orm.py
  • Removed SnowflakeVARIANT TypeDecorator.
  • Removed .with_variant(SnowflakeVARIANT, "snowflake") from
    VrsObject.vrs_object, Allele.state, Extension.value (the
    JSON() / JSONB variants remain).
  • Removed dialect-aware VariationMapping.__table_args__; the
    UniqueConstraint it applied only to Snowflake is gone.
  • Removed now-unused imports (json, urlparse, Dialect,
    UniqueConstraint, TypeDecorator,
    snowflake.sqlalchemy.snowdialect, DEFAULT_STORAGE_URI).
• tests/conftest.py — all_dbs no longer includes "snowflake".
• docs/source/configuration/index.rst,
  docs/source/configuration/storage.rst,
  docs/source/api/index.rst — removed Snowflake bullets, toctree
  entries, and autosummary entries.

[jsstevenson]

what's the gist here? was this previously provided by something else and it's needed for the JWT authentication stuff?

[ehclark]

pyjwt was previously a transitive dependency of snowflake. JWT authorization defined in anyvar.restapi.auth requires. So it was added back in.

[jsstevenson]

lgtm