Refactor authentication middleware to support multiple auth strategies

[Smartdevs17]

Context

Authentication needs to support JWT, API keys, and OAuth with a unified middleware.

Current Limitation

Auth is tightly coupled to JWT with no strategy pattern.

Expected Outcome

• Pluggable auth strategies
• Unified auth middleware
• Strategy-specific validation
• Graceful fallback between strategies

Acceptance Criteria

• Auth strategy interface definition
• JWT strategy implementation
• API key strategy implementation
• OAuth strategy implementation
• Strategy chain with fallback
• Strategy-specific rate limiting
• Auth strategy configuration UI

Technical Scope

• backend/src/middleware/auth.ts - Auth middleware
• backend/src/services/auth/ - Auth strategies
• backend/src/config.ts - Auth configuration
• backend/src/routes/auth.ts - Auth endpoints

[Muyideen-js]

@Muyideen-js has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi! I'm a full-stack developer with TypeScript and Stellar ecosystem experience. I've reviewed this issue and can deliver a quality fix. Please assign this to me!

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[Wilfred007]

@Wilfred007 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Can I work on this task?

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