obs: Add structured JSON logging throughout backend

[YaronZaki]

Problem Statement

All logging uses basic Python logging with f-string interpolation: logger.info(f"Processing position {position_id} for user {wallet_id}"). Logs are unstructured text, making them difficult to parse in log aggregators (Datadog, ELK, CloudWatch).

Evidence

# Throughout codebase — unstructured logging pattern
# quantara/web_app/api/position.py
logger.info(f"Processing position {position_id}")

# quantara/web_app/db/crud/position.py
logger.error(f"Failed to retrieve positions: {str(e)}")

# quantara/web_app/contract_tools/blockchain_call.py
logger.error("Network error fetching account %s: %s", holder_address, exc)

Impact

Medium — observability gap. Cannot filter logs by wallet_id, position_id, or request_id across services. Log aggregation tools cannot parse structured fields. Debugging production issues requires grep on raw text.

Proposed Solution

Add python-json-logger or structlog for structured JSON logging. Include request_id, wallet_id (masked), endpoint, and duration in every log entry. Configure JSON formatter for production, human-readable for development.

Acceptance Criteria

• Structured JSON logging configured using structlog or python-json-logger
• All log entries include: timestamp, level, logger, request_id, message
• Sensitive fields (wallet_id) masked in production logs
• Log format configurable: JSON in production, human-readable in dev
• All existing log calls updated to use structured logger

File Map

• quantara/web_app/utils/logger.py — New: structured logging setup
• quantara/web_app/api/ — update all log calls
• quantara/web_app/db/ — update all log calls
• quantara/pyproject.toml — add structlog or python-json-logger

Dependencies

• Related: REPO-042 (Prometheus metrics complement structured logging)

Testing Strategy

• Unit: Verify log output format in JSON mode and text mode
• Integration: Run request through API, verify log output contains expected structured fields

Security Considerations

Ensure wallet_id is masked/hashed in production logs. Never log private keys, secrets, or full API responses. Structured logging should not increase PII leakage.

Definition of Done

• Code implemented and peer-reviewed
• All log calls updated
• PR linked and merged

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Labels: observability
Priority: Medium
Difficulty: Intermediate
Estimated Effort: 4h

[shadrach68]

I have 3+ years of experience with Backend development and have reviewed the code. I can submit the PR within 48hrs.