feat(collection): optional progress bar for Collection.upsert (#10)

[srijanarya]

Closes #10, scoped to upsert; create_index progress is left for a follow-up as discussed in the issue.

Adds an opt-in show_progress: bool = False parameter to Collection.upsert. When enabled:

• Uses tqdm.auto, so it picks a notebook widget or a terminal bar automatically (the notebook-vs-shell problem raised in the issue).
• Reports a known total when records is Sized (list, tuple, ...), and falls back to an indeterminate bar for generators and other unsized iterables.
• tqdm is an optional extra (pip install vecs[progress]), not a new hard dependency, so pip install vecs is unchanged. Calling with show_progress=True without tqdm installed raises an ImportError with the install hint.
• Default behaviour (show_progress=False) is unchanged.

Tests added in src/tests/test_upsert_progress.py for: default creates no progress object, sized input reports the total and updates per 500-record chunk, generator input is indeterminate and still updates per chunk, and the ImportError path when tqdm is absent. Full suite run locally against supabase/postgres:15.1.1.78: 44 passed; the 3 test_adapters.py text-embedding tests fail identically on main without the sentence-transformers extra installed.