Fix #769: Implement Distributed Write-Ahead Journaling

[SatyamPandey-07]

Key Features Implemented:
Write-Ahead Journaling (WAJ): Created

models/WriteJournal.js
to serve as a persistent, high-throughput buffer for all pending state changes. This ensures that no mutation is lost and provides a scale-out path for database writes.
Causal Consensus Engine: Developed

services/consensusEngine.js
using Vector Clocks. This engine determines the relationship between concurrent updates, automatically applying causal successors and flagging divergent "concurrent" edits for resolution.
Non-Blocking Journal Interceptor: Implemented

middleware/journalInterceptor.js
which can transparently intercept standard REST mutations and redirect them to the journal, returning an optimistic 202 Accepted response to the user.
Base Repository Overhaul: Massively refactored

repositories/baseRepository.js
to support { deferred: true } CRUD operations. This allows any repository in the system to leverage journaling with zero-config overhead.
Asynchronous Journal Applier: Built

jobs/journalApplier.js
, a background worker that continuously flushes the journal buffer, applies consensus logic, and finalizes records into the main database.
State Differential Utility: Created

utils/diffEngine.js
to perform deep object comparison and generate patches, minimizing the data payload needed for resolution.
Optimistic UI Service Integration: Updated

transactionService.js
and

expenseService.js
to return "Journaled" tokens, allowing the frontend to show immediate feedback while the write matures in the background.
Manual Conflict Resolution API: Added

routes/conflicts.js
providing endpoints to browse unresolvable concurrent edits and choose the "winning" state (Source of Truth selection).
Distributed Consistency Metadata: Enhanced

models/Workspace.js
with journalVersionEpoch to maintain a global sequence of state transitions across different nodes.
Consensus Integrity Validation: Developed

tests/consensusLogic.test.js
to rigorously test causal ordering, ensuring that older writes never overwrite newer data.
closes #769

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🎉 Thanks for the PR, @SatyamPandey-07!

We really appreciate you taking the time to contribute to ExpenseFlow! 💙

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