feat(combos): add custom context_length configuration for model combos

[mahdiwafy]

Summary

Adds support for configuring and advertising an optional custom context_length on model combos.

Changes

1. Schema & DB Repo: Expose optional context_length column on combos table (SCHEMA_VERSION bumped to 2).
2. API: Accept and update context_length in /api/combos CRUD endpoints.
3. Model Listing: Expose context_length in /v1/models output for combo entries when set.
4. Dashboard UI: Add an optional "Advertised Context Length" input field in Combo create/edit modal and render context length badges on combo cards.

[mahdiwafy]

Heads up: this PR touches src/app/api/combos/route.js and src/lib/db/repos/combosRepo.js to add the context_length field — and #61 touches the exact same two files/functions to add the alias field (both insert into the same SQL statements). Whichever of these two merges first will cause a straightforward-but-real text conflict for the other on merge.

Not a blocker, just flagging it so whoever merges second knows to expect it and can resolve by keeping both fields (verified locally: manually merging both changes together works cleanly, no functional conflict — just needs both columns/params present in the same INSERT/UPDATE).

[mahdiwafy]

Superceded and merged locally into the main working branch (fix/claude-5-models). Closing to clean up PR list.