perf(db): implement composite B-Tree database indexes for filtering and pagination (#105)

[Decuayer]

🎯 Jira / Task Reference

• Jira Issue: SDDC-69
• GitHub Issue: Closes [SDDC-69] Implement Composite B-Tree Database Indexes for Optimized Server-Side Filtering, Pagination, and Search #105
• Component: database

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📝 Summary of Changes

Implements composite B-Tree database indexes across the Incidents, Teams, and Users tables in accordance with NFR-01, NFR-02, and Section 4.2 of the Design Document:

• Incidents Index: (CreatedAt DESC, Status, Category) (IX_Incidents_CreatedAt_Status_Category) to optimize chronological incident queue pagination and multi-parameter status/category filtering.
• Teams Index: (Status, TeamName) (IX_Teams_Status_TeamName) for fast idle team lookup and name prefix searching.
• Users Index: (RoleType, Department, FirstName, LastName) (IX_Users_RoleType_Department_FirstName_LastName) to optimize directory filtering by organizational unit and role-based searching.
• EF Core Migration: Generated and applied 20260831152146_AddCompositeIndexesForFilteringAndPagination.cs.
• Automated Tests: Added metadata constraint unit tests in DbContextConstraintTests.cs validating index names, property sequences, and descending ordering.

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🏷️ Type of Change

• 🚀 Feature: New functionality or operational workflow
• 🐛 Bug Fix: Non-breaking fix for an identified defect
• ♻️ Refactor: Code restructuring without behavior alteration
• ⚡ Performance: Speed, query, or memory consumption optimization
• 🛠️ Chore / Infra: Dependencies, Docker, CI/CD, or tooling updates
• 📖 Documentation: README, API specs, or code documentation updates
• ⚠️ Breaking Change: Requires schema migration or API contract changes

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🧪 Testing & Verification

Automated Tests

• Backend unit/integration tests executed: dotnet test (118/118 passed)
• Linter & formatting passed: dotnet format --verify-no-changes

Manual Test Steps & PostgreSQL Execution Plans

1. Applied migration via dotnet ef database update.
2. Verified presence of target indexes in PostgreSQL system catalog via pg_indexes.
3. Executed EXPLAIN (ANALYZE, BUFFERS) verifying switch from Seq Scan to Index Scan:
   • Incidents status/category query: Index Scan using "IX_Incidents_CreatedAt_Status_Category" (~1.26ms)
   • Teams availability query: Index Scan using "IX_Teams_Status_TeamName" (~1.19ms)
   • Users role & department query: Index Scan using "IX_Users_RoleType_Department_FirstName_LastName" (~1.34ms)

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📷 Screenshots / Evidence (Required for UI / API changes)

Query	Execution Plan / Verification
Incidents Filter	Index Scan using "IX_Incidents_CreatedAt_Status_Category" on "Incidents"
Teams Availability	Index Scan using "IX_Teams_Status_TeamName" on "Teams"
Users Directory	Index Scan using "IX_Users_RoleType_Department_FirstName_LastName" on "Users"

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📋 Pre-Merge Checklist

• My code adheres to the project's Clean Architecture & style guidelines.
• I have self-reviewed my own code before requesting review.
• Database migrations are backwards-compatible (if applicable).
• Sensitive secrets / tokens are NOT committed (verified .env exclusions).
• Jira status updated to In Review.