Add API key log redaction

[neuromechanist]

Type: Security
Priority: P1 (High severity - prevents credential leakage)
Effort: 1 hour
Phase: 3

Description:
API keys might accidentally be logged in debug output or error messages. Need automatic redaction to prevent exposure in centralized logging systems.

Security Impact: HIGH - Prevents credential exposure in logs

Acceptance Criteria:

• Custom log formatter that redacts API keys
• Pattern: sk-or-v1-[0-9a-f]{64} → sk-or-v1-***[redacted]
• Applied to all loggers (root logger configuration)
• Test that API keys are redacted in log output
• Does not affect actual API calls (only logging)

Implementation: Create custom SecureFormatter class extending logging.Formatter

Related Issues: Part of security hardening with #64, #66, #67, #68
Branch: feature/issue-64-68-security-hardening

[neuromechanist]

Addressed in PR #248 (Security hardening). SecureFormatter wired up in app startup, with tests in test_logging.py.