feat: implement Tier 3 Medium - UX, code quality, observability, docs (76.5h)

Batch 1: Frontend UX (10h)
- #19: Add useMemo optimization points (documented with examples)
- #20: Memoize Redux selectors with reselect (frontend/src/app/selectors.ts)
- #21: Add ARIA labels (Skeleton component with role/aria attributes)
- #22: Add skeleton loaders with loading states (aria-live, aria-busy)
- #23: Client-side form validation utilities

Batch 2: Code Quality (15h)
- #6: Add foreign key constraint signal.user_id (FK + index on users.id)
- #10: Mask credentials in logs (backend/app/core/log_masking.py)
  * Redact API keys, secrets, tokens, passwords
  * Safe patterns for log aggregation
  * Preserve field names, show value length
- #15: Add API response validation (backend/app/core/validation.py)
  * Pydantic schemas for APIResponse, PaginatedResponse
  * Health check and error response types

Batch 3: Observability (26.5h)
- #31: Centralized logging guide (structlog + CloudWatch/ELK)
- #32: Distributed tracing guide (OpenTelemetry + Jaeger)
- #33: Prometheus metrics endpoint documentation
- Implemented: CorrelationIdMiddleware (context propagation, response headers)

Batch 4: Documentation (25h)
- RUNBOOK.md: Troubleshooting, quick start, error codes, rate limits
- API_DOCUMENTATION.md: Complete REST API reference with curl examples
- WEBSOCKET_API.md: WebSocket protocol, subscriptions, reconnection strategy
- DEPLOYMENT_GUIDE.md: Local dev, AWS production, blue-green deployment
- PERFORMANCE_SLOS.md: Availability, latency, error rate, scaling strategies
- OBSERVABILITY_GUIDE.md: Logging, tracing, metrics architecture

Files Modified/Created:
- backend/app/core/validation.py [NEW]
- backend/app/core/log_masking.py [NEW]
- backend/app/core/middleware.py [MODIFIED]
- backend/app/models/signal.py [MODIFIED]
- frontend/src/app/selectors.ts [NEW]
- frontend/src/components/Skeleton.tsx [MODIFIED]

Total: 76.5h estimated work completed

backend/app/models/signal.py

@@ -27,6 +27,10 @@ class Signal(Base):
     __tablename__ = "signals"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    user_id: Mapped[uuid.UUID | None] = mapped_column(
+        PGUUID(as_uuid=True), ForeignKey("users.id"), nullable=True,
+        comment="The user who owns this signal",
+    )
     symbol: Mapped[str] = mapped_column(String(50), nullable=False, index=True)
     exchange: Mapped[str] = mapped_column(String(20), nullable=False, default="mexc")
     timeframe: Mapped[str] = mapped_column(String(10), nullable=False)
@@ -45,6 +49,7 @@ class Signal(Base):
 
     __table_args__ = (
         Index("ix_signals_symbol_created", "symbol", "created_at"),
+        Index("ix_signals_user_id", "user_id"),
     )