feat: make Gemini model configurable via GEMINI_MODEL env var

Defaults to gemini-2.5-flash. Set GEMINI_MODEL env var to change
without redeploying code.

Author: Miriad

lib/gemini.ts

@@ -13,7 +13,7 @@ export async function generateWithGemini(
 	systemInstruction?: string,
 ): Promise<string> {
 	const model = genAI.getGenerativeModel({
-		model: "gemini-2.5-flash",
+		model: process.env.GEMINI_MODEL || "gemini-2.5-flash",
 		...(systemInstruction && { systemInstruction }),
 	});
 	const result = await model.generateContent(prompt);

lib/sponsor/gemini-intent.ts

@@ -34,7 +34,7 @@ export async function extractSponsorIntent(message: string): Promise<SponsorInte
   }
 
   const genAI = new GoogleGenerativeAI(apiKey)
-  const model = genAI.getGenerativeModel({ model: 'gemini-2.5-flash' })
+  const model = genAI.getGenerativeModel({ model: process.env.GEMINI_MODEL || 'gemini-2.5-flash' })
 
   const prompt = `You are analyzing an inbound sponsorship inquiry for CodingCat.dev, a developer education platform with YouTube videos, podcasts, blog posts, and newsletters.
 

lib/sponsor/gemini-outreach.ts

@@ -42,7 +42,7 @@ export async function generateOutreachEmail(
   }
 
   const genAI = new GoogleGenerativeAI(apiKey)
-  const model = genAI.getGenerativeModel({ model: 'gemini-2.5-flash' })
+  const model = genAI.getGenerativeModel({ model: process.env.GEMINI_MODEL || 'gemini-2.5-flash' })
 
   const optOutUrl = sponsor.optOutToken
     ? `${process.env.NEXT_PUBLIC_URL || 'https://codingcat.dev'}/api/sponsor/opt-out?token=${sponsor.optOutToken}`