Speed up existing devcontainer database lookups

[n4p68vc6p9-hub]

/claim #24

Related bounty issue: https://github.com/daytonaio/devcontainer-generator/issues/24

Summary

• Return cached database results before traversing the GitHub repository when a devcontainer entry already exists.
• Cache positive check_url_exists lookups for a short TTL (DEVCONTAINER_DB_CACHE_SECONDS, default 300) to avoid repeated Supabase queries for frequently requested repositories.
• Narrow the Supabase select list to only the fields needed for the response.
• Extract shared result rendering into render_devcontainer_result so database, repository, and generated responses use the same UI path.

Why

Issue #24 asks to speed up returning existing database entries and reduce redundant database queries. The previous flow checked the database but still fetched repository context before returning existing records, so existing entries still paid the slower GitHub traversal path. This PR returns existing database records immediately and caches repeated positive lookups.

Notes

• regenerate=true bypasses the cache and refreshes the stored result.
• Cache entries are invalidated after insert/regeneration.