feat: add per-provider HTTP proxy support and fix model re-creation bug

Add configurable HTTP proxy for each upstream provider, using Bun's native
fetch proxy support. Also fix a bug where re-creating a model with the same
name after soft-deletion would fail due to unique constraint violation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pescn

backend/drizzle/0016_ambiguous_jigsaw.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "providers" ADD COLUMN "proxy_url" varchar(255);--> statement-breakpoint
+ALTER TABLE "providers" ADD COLUMN "proxy_enabled" boolean DEFAULT false NOT NULL;