[BUG] Oracle multi-DB resilience: connection toggle + cold start warm-up (P0)

[cct08311github]

Fix Oracle-specific stability issues in multi-database deployment (MS SQL + Oracle):

• BUG-7: Add Enabled toggle per DB connection in appsettings.json — graceful degradation when Oracle is unreachable
• BUG-8: Oracle first query ~60s delay — implement connection pool warm-up + connection string tuning

Production impact: app crashes on startup if Oracle DB is down; first user query to Oracle takes ~60 seconds.

[cct08311github]

Fixed in PR #17 (merged into dotnet8). Covered by BUG-7 (DB Enabled toggle) and BUG-8 (Oracle warm-up service).