feat: add bedrock native passthrough, generic passthrough, provider docs, and managed backend fixes

Adds bedrock_native.rs (Converse/InvokeModel with SigV4) and
generic_passthrough.rs catch-all for Translate mode. Adds comprehensive
provider reference docs (docs/providers/, docs/ENDPOINTS.md). Fixes
managed backend admin UI (BackendForm, ManagedBackendsSection) and
admin route/model handler issues. Adds automated model pricing update
workflow (scripts/update_pricing.py, .github/workflows/update-pricing.yml).

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

.github/workflows/update-pricing.yml

@@ -0,0 +1,35 @@
+name: Update Model Pricing
+
+on:
+  schedule:
+    # Every Monday at 06:00 UTC
+    - cron: '0 6 * * 1'
+  workflow_dispatch:
+
+jobs:
+  update-pricing:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Fetch and update pricing
+        run: python scripts/update_pricing.py
+
+      - name: Commit if changed
+        run: |
+          git diff --quiet assets/model_pricing.json && {
+            echo "Pricing already up to date."
+            exit 0
+          }
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git add assets/model_pricing.json
+          git commit -m "bot: update model pricing from LiteLLM [skip ci]"
+          git push