feat: Add API key authentication for backup tool

- Add get_api_key() and require_api_key() to auth.py
- Add generic key-value settings storage to SettingsStore
- Add test_api_key_auth to verify the implementation
- Uses secrets.compare_digest for timing-safe comparison
- Auto-generates API key on first use and stores in settings DB

.fusion/memory/2026-05-11.md

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+# Daily Memory 2026-05-11
+
+<!-- Append running observations, open loops, and day-to-day notes here. Promote evergreen facts to MEMORY.md. -->

.fusion/memory/DREAMS.md

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+# Memory Dreams
+
+<!-- Periodic synthesized patterns from daily notes. Promote durable lessons to MEMORY.md. -->

.fusion/memory/MEMORY.md

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+# Project Memory
+
+<!-- This file stores durable project learnings. Agents consult and update it during triage and execution. -->
+
+## Architecture
+
+<!-- Key architectural patterns, module boundaries, and design decisions -->
+
+## Conventions
+
+<!-- Project-specific coding standards, naming patterns, file organization -->
+
+## Pitfalls
+
+<!-- Known issues, common mistakes, and things to avoid -->
+
+## Context
+
+<!-- Important background information, dependency constraints, deployment notes -->