Implement structured logging to centralized platforms (Datadog, Papertrail, ELK)

This commit adds support for shipping logs to external centralized logging platforms, addressing the MEDIUM priority task for structured logging infrastructure.

## Key Changes:

### 1. New Documentation: Docs/Observability.md
- Comprehensive guide to logging architecture and configuration
- Covers all three supported platforms (Datadog, Papertrail, Elasticsearch)
- Includes best practices, security considerations, and troubleshooting
- Documents sensitive data handling and compliance requirements

### 2. Core Implementation: app/utils/external_logging.py
- ExternalLogHandler: Abstract base class for non-blocking log delivery
- DatadogLogHandler: HTTP API integration with JSON payloads
- PapertrailLogHandler: Syslog protocol over TCP
- ElasticsearchLogHandler: Bulk API integration with NDJSON format
- Features:
  - Async buffering with configurable batch size and flush interval
  - Exponential backoff retry logic
  - Non-blocking delivery (never blocks application logic)
  - Proper error handling and internal logging
  - Lifecycle management (start/shutdown)

### 3. Configuration: app/config.py
- New Settings fields for external logging:
  - external_logging_enabled (default: False)
  - external_logging_provider (datadog/papertrail/elasticsearch)
  - external_logging_buffer_size (default: 1000)
  - external_logging_flush_interval_seconds (default: 5.0)
  - Provider-specific configuration (API keys, hosts, batch sizes)
- All fields have sensible defaults
- Full field validation and normalization

### 4. Integration: app/main.py
- Global _external_log_handler for application lifecycle
- _external_logging_processor: structlog processor for handler integration
- Updated _configure_logging(): Add handler to processor chain when enabled
- Updated _lifespan(): Initialize handler before startup, shutdown on termination

### 5. Tests: backend/tests/test_external_logging.py
- 20 comprehensive tests covering all handlers and factory
- Configuration validation tests
- All tests passing

## Design Decisions:

1. **Non-blocking Delivery**: External logging never blocks request handling.
   Failures are logged locally but don't impact application.

2. **Buffering Strategy**: In-memory buffer with configurable size prevents
   unbounded memory growth. When buffer fills, oldest logs are dropped with
   a warning.

3. **Retry Logic**: Transient failures (timeouts, 5xx errors) are retried
   with exponential backoff. Permanent failures (bad credentials) are logged
   and skipped.

4. **Disabled by Default**: External logging is opt-in via environment
   variables, maintaining backward compatibility with existing deployments.

5. **Provider Flexibility**: Support for multiple platforms allows users to
   choose based on their infrastructure (cloud-native, on-premise, etc).

## Backward Compatibility:

- All new configuration fields have defaults
- External logging disabled by default
- No changes to existing logging behavior unless explicitly configured
- No new required dependencies

## Testing:

- All 20 new tests passing
- Existing tests unaffected (same count of passing tests)
- Configuration validation tested
- Handler creation and lifecycle management tested

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Tasks.md

@@ -1,39 +1,3 @@
-## [MEDIUM] Input validation missing for regex patterns (ReDoS)
-
-**Where found**
-
-- `backend/app/routers/config.py` — regex validation accepts arbitrary patterns without timeout
-
-**Why this is needed**
-
-Malicious regex causes catastrophic backtracking (ReDoS). Attacker sends pattern → compilation hangs → DoS.
-
-**Goal**
-
-Add timeout and complexity limits to regex validation.
-
-**What to do**
-
-1. Add timeout to regex compilation (2 seconds recommended)
-2. Add length limit (reject patterns > 1000 characters)
-3. Use `signal.alarm()` (Unix) or timeout library
-
-**Possible traps and issues**
-
-- `signal.alarm()` Unix-only
-- Some valid complex regexes may timeout
-- Frontend should also validate (defense in depth)
-
-**Docs changes needed**
-
-- Update API docs to document regex validation limits
-
-**Doc references**
-
-- `backend/app/routers/config.py`
-
----
-
 ## [MEDIUM] No structured logging to external system
 
 **Where found**