Multi-source filters + quality tiers

What's new

Shape your source mix server-side. source now accepts a comma-separated list — GET /v1/questions?source=mintaka,opentdb returns questions from any of the listed sources. And a new exclude_source drops sources you don't want in one call: ?exclude_source=entityq filters out a noisy auto-generated source instead of pulling everything and discarding half client-side. Both work on /v1/questions and /v1/questions/random.
A sharper default quality bar — with an escape hatch. quality=high (the default) now returns only our cleanest, most broadly-useful questions. The previous behavior — a wider pool that includes more niche and narrowly-scoped questions — is available as the new quality=standard. quality=all is unchanged (the full pool, including questions flagged for review). If you want more volume than the new high, switch to quality=standard.
Same capabilities in the SDK and MCP. @quizbase/client and the MCP server's quizbase_list / quizbase_random tools gained multi-source, exclude_source, and quality=standard.

source and exclude_source are both lists; if you pass both, exclusions are applied after the includes.
The three quality tiers nest: high ⊆ standard ⊆ all. Pick high for "just the best", standard for breadth, all for everything (each question then carries a quality field so you can tell which were flagged).
Everything is additive — a single source=mintaka, and existing requests, behave exactly as before. The one behavior change is that the default quality=high is now stricter; pass quality=standard to keep the old, broader result set.