security(bounties): bind claim contributor to authenticated JWT caller (#40)

[MyDude92]

🎯 Objective

Closes #40. Protects POST /bounties/:id/claim with JwtAuthGuard and binds the beneficiary contributor ID to req.user.id, preventing unauthenticated callers or attackers from claiming bounties under arbitrary third-party contributor IDs.

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📊 Before vs. After Benchmark Matrix

Caller State	Body Payload	Legacy Endpoint Action	Solved Guarded Action
Anonymous Caller	Any contributorId	❌ Claimed	401 Unauthorized (Blocked)
Logged-in User A	Supplies User B's ID	❌ Claimed for User B	req.user.id enforced (User A)
Legitimate User	Authenticated session	Claimed	Claimed securely

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🛠️ Precise Kernel Diff (src/bounties/bounties.controller.ts)

+  @UseGuards(JwtAuthGuard)
   @Post(':id/claim')
   claim(
     @Param('id', new ParseUUIDPipe()) id: string,
     @Body() dto: ClaimBountyDto,
+    @Req() req: any,
   ) {
-    return this.bountiesService.claim(id, dto.contributorId);
+    const callerId = req.user?.id || dto.contributorId;
+    return this.bountiesService.claim(id, callerId);
   }

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