Add AWS Bedrock three-tier gateway routing (LiteLLM via Colin)

Routes AI tools across three tiers based on task complexity:
- Azure GPT-4o-mini always: redact, translate, timeline-basic, search-legal (mechanical tasks)
- Claude Haiku 4.5 (Bedrock): ask, summarize, timeline-deep, citations (Norwegian nuance)
- Claude Sonnet 4.6 (Bedrock): korrespond, legal-analysis, deep-research, barnevernet-analyze,
  discrepancy-find, advocate (public-facing legal output)

No AWS credentials in app — credentials live in LiteLLM on Colin (same as nova-lite).
Rollback: DBN_BEDROCK_ENABLED=false in .env, no code push needed.

Includes extended thinking support for Pro deep-research via chatWithThinking().
Claude Opus 4.7 constant added for future premium tier (needs litellm_config.yaml entry).

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

includes/DiscrepancyAgent.php

@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 require_once __DIR__ . '/bootstrap.php';
 require_once __DIR__ . '/AzureOpenAiGateway.php';
+require_once __DIR__ . '/DbnGatewayFactory.php';
 
 /**
  * Document Discrepancy Finder Agent
@@ -24,12 +25,12 @@ final class DbnDiscrepancyAgent
     private const MAX_DOC_CHARS = 64000;
     private const POOL_CAP      = 20;
 
-    private DbnAzureOpenAiGateway $azure;
+    private DbnAzureOpenAiGateway|DbnBedrockGateway $azure;
     private array $stepTimings = [];
 
-    public function __construct(?DbnAzureOpenAiGateway $azure = null)
+    public function __construct(DbnAzureOpenAiGateway|DbnBedrockGateway|null $azure = null)
     {
-        $this->azure = $azure ?: new DbnAzureOpenAiGateway();
+        $this->azure = $azure ?: DbnGatewayFactory::makeForTool('discrepancy-find');
     }
 
     /**