feat(api): add ledger entries read endpoint

[Sam-24-dev]

Closes #149

Summary

• Add a read-only tenant-scoped Ledger entries directory.
• Return an explicit public projection with Decimal values serialized as strings.
• Add signed, session-bound keyset pagination and authorization regression coverage.

Changes

File	Change
backend/api/ledger_views.py	Add the Ledger entries API view.
backend/api/urls.py	Route /api/v1/ledger-entries/.
backend/api/tests.py	Cover authorization, projection, and cursor behavior.

Testing

• manage.py check
• manage.py makemigrations --check --dry-run
• manage.py test api.tests --verbosity 0 (86 passed)
• manage.py test --verbosity 0 (319 passed, 14 expected skips)
• git diff --check

Contributor checklist

• Linked an approved issue
• Exactly one type:* label
• Conventional commit
• No Co-Authored-By trailer

[Copilot]

🟢 Approval recommended

The endpoint implementation follows existing cursor/auth patterns in the codebase and is backed by targeted and suite-wide passing tests covering the key acceptance criteria.

Pull request overview

Adds a new tenant-scoped, session-authenticated, read-only API endpoint for listing Ledger entries in the active workspace, including a strict public projection and session-bound signed keyset pagination per issue #149.

Changes:

• Introduces GET /api/v1/ledger-entries/ with a fixed output projection and no-store responses.
• Implements opaque, signed, session-bound keyset cursors ordered by occurred_on then pk.
• Adds API tests covering auth/membership rules, projection (including Decimal-as-string), cursor validation, and tamper/expiry handling.

File summaries

File	Description
backend/api/ledger_views.py	New read-only ledger entries listing view with signed, session-bound keyset cursors and strict query contract.
backend/api/urls.py	Routes /api/v1/ledger-entries/ to the new list view.
backend/api/tests.py	Adds authorization, projection, cursor validation, and cursor storage pruning/cap tests for the new endpoint.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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