Tighten Sentry init (`traces_sample_rate`, profiling)

[YaronZaki]

• Difficulty: good-first-issue · Effort: XS · Impact: observability cost

• Problem Statement
  quantara/web_app/api/main.py calls:

  sentry_sdk.init(
      dsn=os.getenv("SENTRY_DSN"),
      traces_sample_rate=1.0,
      _experiments={"continuous_profiling_auto_start": True},
  )

  Two problems: 100% sampling is a temporary expedient that's disastrous
  once traffic climbs. _experiments is an unstable surface; Sentry has
  no compatibility guarantee on those keys.

• Why It Matters
  At 50k req/day, a 100% sample rate sends 50k spans/hour into Sentry, a
  monthly bill north of five figures within weeks of go-live.

• Expected Outcome
  Configurable samples from env (SENTRY_TRACES_SAMPLE_RATE default 0.1,
  SENTRY_PROFILES_SAMPLE_RATE default 0.1); _experiments removed.

• Acceptance Criteria

  • Process startup logs the active sample rate.
  • _experiments removed from init call.
  • docs/environment_variables.md updated with both new env vars.
  • Trace volume test (synthetic burst of 100 req in 1 min) confirms ratio <0.1 in Sentry.

• Implementation Notes

  sentry_sdk.init(
      dsn=os.getenv("SENTRY_DSN"),
      traces_sample_rate=float(os.getenv("SENTRY_TRACES_SAMPLE_RATE", "0.1")),
      profiles_sample_rate=float(os.getenv("SENTRY_PROFILES_SAMPLE_RATE", "0.1")),
  )

• Files / Modules Affected

  • quantara/web_app/api/main.py
  • quantara/web_app/config_validator.py
  • docs/environment_variables.md

• Dependencies: standalone.

[kevinnft]

Opened PR tightening Sentry sample rates and removing _experiments:

#358