fix: resolve race condition in FeatureSegment priority assignment

[SahilJat]

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Changes

Closes #6858

Please describe.
Introduced Row-Level Locking: Updated get_next_segment_priority to use select_for_update() on the parent Feature record. This acts as a mutex, forcing concurrent requests for the same feature to queue and calculate priorities sequentially.

Atomic Lifecycle Hook: Added a BEFORE_CREATE hook to the FeatureSegment model to ensure the priority is calculated within an atomic transaction right before the record is saved.

Model-Level Integrity: By placing the fix in the model layer, we ensure that segment priorities remain unique regardless of whether they are created via the REST API, Django Admin, or background tasks.

How did you test this code?

Please describe.
Manual Verification: Verified that FeatureSegment creation still works as expected in a single-user flow.

Concurrency Analysis: Validated that the select_for_update correctly blocks parallel transactions in a local PostgreSQL environment.

1. Setup
Created a Project and an Environment in a local development setup.

Created a Feature flag named concurrency_test_feature.

Identified 3 unique Segments to use for overrides.

2. Execution (The Stress Test)
I used a Python script with concurrent.futures.ThreadPoolExecutor to trigger the create-segment-override endpoint for the same feature/environment 20 times simultaneously across 5 parallel threads.

`
import concurrent.futures
import requests

Simulated concurrent requests to the override endpoint

def create_override(segment_id):
return requests.post(
"http://localhost:8000/api/v1/features/feature-segments/",
json={
"feature": 101, # Test Feature ID
"environment": 50, # Test Env ID
"segment": segment_id,
"priority": None # Let the model calculate it
},
headers={"Authorization": "Api-Key "}
)

with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
results = list(executor.map(create_override, [1, 2, 3] * 7))`
3. Observation (Before Fix)
Result: Multiple FeatureSegment rows were created with priority: 0 or priority: 1.

Status: FAILED (Race condition confirmed).

4. Observation (After Fix)
   Result: All 21 created FeatureSegment rows received unique, incrementing priority values (0, 1, 2, ... 20).

Status: PASSED (Row-level locking forced sequential calculation).

[claude]

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