fix(quickstart): add -p flag to avoid error from missing PostgreSQL

[brunoguerios]

Summary

• Running cargo run --release --example quickstart from the workspace root fails with ld: library 'pq' not found on machines without PostgreSQL installed
• Cargo resolves the full workspace graph, pulling pq-sys → diesel → tycho-storage/tycho-indexer into the link step even though the quickstart has no database dependency
• Fix: add -p tycho-simulation to all documented commands so the build is scoped to that crate's dependency tree, which does not include pq-sys

Alternative considered

• Documenting brew install postgresql / apt install postgresql as a prerequisite. Rejected because the quickstart targets simulation consumers (solver/routing developers) who will never run the indexer and have no reason to install a PostgreSQL client. The -p flag is also the semantically correct Cargo idiom for building a single workspace member.

Also includes

• Added TYCHO_API_KEY to all code blocks — it was previously undocumented, causing a confusing Invalid authentication key error on first run

[claude]

Claude Code Review

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[zizou0x]

Nice catch! LGTM, thanks!

[brunoguerios]

Hey @zizou0x - thanks for reviewing it!
I brought the branch up to date with latest changes on main, so it's ready to be merged ✅
Even though you approved the changes, seems that a few workflows are still pending.
Let me know if there's anything else you require from me to get this through or if you'll take it from here.
Cheers 🍻