feat: Centralize logging setup with structlog integration

[khvn26]

Closes #31

What changed

Logging setup moves from Django settings into flagsmith-common, running before Django loads. Both stdlib and structlog now route through a single ProcessorFormatter, so all output (app code, structlog, Gunicorn error logs) shares the same format and ISO timestamps.

Key additions

• setup_logging() in common.core.logging — configures stdlib via dictConfig, then wires structlog through stdlib using LoggerFactory. Accepts application_loggers for per-package level control and extra_foreign_processors for extending the foreign_pre_chain.
• _SentryFriendlyProcessorFormatter helps keep JSON logs compatible with Sentry's error grouping.
• sentry_processor in common.core.sentry sets Sentry context from event dicts.
• application_loggers allowlist — root logger stays at WARNING to suppress third-party noise; listed app loggers get the requested level. DEBUG overrides everything. Configurable via APPLICATION_LOGGERS env var.
• make_gunicorn_access_processor() in common.gunicorn.logging ports former GunicornAccessLogJsonFormatter logic to a structlog processor. Configurable via ACCESS_LOG_EXTRA_ITEMS env var.
• Gunicorn error logs propagate to root — no separate handler/formatter needed.
• Gunicorn access logs — JSON mode propagates to root for structured output via the access processor; generic mode keeps Gunicorn's own %(message)s handler for pure CLF output.
• TTY-aware colors — ConsoleRenderer only enables ANSI colors when stdout is a TTY.
• sentry-sdk added as an explicit dependency in the common-core group.
• environs used in ensure_cli_env for env var parsing.

What it removes

• GunicornAccessLogJsonFormatter and GunicornAccessLogJsonRecord — replaced by the foreign_pre_chain processor.
• Separate formatter management for Gunicorn error logs.
• LOGGING_DEFAULT_GENERIC_FORMAT constant — ConsoleRenderer handles generic formatting.
• PROMETHEUS_HISTOGRAM_BUCKETS and PROMETHEUS_HTTP_SERVER_RESPONSE_SIZE_HISTOGRAM_BUCKETS Django settings — histogram buckets now use library defaults directly, removing import-time Django settings access that prevented pre-Django logging setup.

What it keeps

• JsonFormatter and JsonRecord — JsonRecord is still the schema definition used by map_event_to_json_record.
• Gunicorn access log CLF format in generic mode.
• Gunicorn constants (WSGI_EXTRA_SUFFIX_TO_CATEGORY, wsgi_extra_key_regex) in their original location.

Migration guide for flagsmith/flagsmith

1. Replace the LOGGING dict in settings/common.py

# Remove the entire LOGGING = { ... } block (lines ~615-668) and replace with:
LOGGING = {"version": 1, "disable_existing_loggers": False}

Django's dictConfig(LOGGING) call during django.setup() becomes a no-op.

2. Set env vars

APPLICATION_LOGGERS=app,features,integrations,task_processor,app_analytics,webhooks,gunicorn
ACCESS_LOG_EXTRA_ITEMS={flagsmith.route}e,{origin}i,{access-control-allow-origin}o

3. Delete api/util/logging.py

It duplicates JsonFormatter which now lives in common.core.logging.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.47%. Comparing base (398c83c) to head (852780f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
+ Coverage   96.12%   96.47%   +0.35%     
==========================================
  Files          92       97       +5     
  Lines        3222     3550     +328     
==========================================
+ Hits         3097     3425     +328     
  Misses        125      125              

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