[DOCS] Update configuration.md and local.settings.json.example for Key Vault

[artcava]

Overview

Update documentation to reflect the new Key Vault-based configuration introduced by the LinkedIn token auto-refresh feature. Ensures any developer setting up XPoster locally or in a new environment can follow a clear, up-to-date guide.

Depends on: #54 #55

Tasks

• Update src/local.settings.json.example:
  • Replace LINKEDIN_ACCESS_TOKEN plain value with a comment explaining it is now a Key Vault Reference in Azure
  • Add LINKEDIN_CLIENT_ID, LINKEDIN_CLIENT_SECRET, LINKEDIN_REFRESH_TOKEN with instructions
  • Add KEYVAULT_URI and LinkedInTokenRefreshSchedule
• Update docs/configuration.md:
  • Add Key Vault References section explaining the @Microsoft.KeyVault(...) syntax
  • Document how to set up local development with az login + DefaultAzureCredential
  • List all new environment variables with type, required/optional, and where to obtain each value
• Update docs/analysis-linkedin-token-auto-refresh.md:
  • Mark Open Questions as resolved
  • Update decisions taken: Key Vault shared, client_secret in KV, cron 0 0 9 1 * *

New Variables to Document

Variable	Storage	Description
LINKEDIN_CLIENT_ID	Key Vault	LinkedIn app client ID
LINKEDIN_CLIENT_SECRET	Key Vault	LinkedIn app client secret
LINKEDIN_ACCESS_TOKEN	Key Vault Reference	Current access token (auto-refreshed)
LINKEDIN_REFRESH_TOKEN	Key Vault	Refresh token (valid 365 days)
LinkedInTokenRefreshSchedule	App Setting	Cron: 0 0 9 1 * *
KEYVAULT_URI	App Setting	https://xposter-kv.vault.azure.net/

References

• Analysis document
• docs/configuration.md

[github-actions]

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[artcava]

Partial completion — docs updated in PR #64

The following tasks from this issue have been completed as part of PR #64 (feature/55-migrate-linkedin-credentials-keyvault):

• src/local.settings.json.example updated: LINKEDIN_CLIENT_ID, LINKEDIN_CLIENT_SECRET, KEYVAULT_URI added with inline comments explaining the production Key Vault Reference syntax vs. local raw-value approach
• docs/configuration.md updated: LinkedIn table extended with new variables; new Azure Key Vault References subsection added documenting reference syntax, prerequisites (Managed Identity + RBAC), and Azure Portal verification steps

Remaining tasks — deferred

The following tasks depend on the token refresh chain (#56 → #57 → #58) and are deferred until that work is unblocked:

• Add LINKEDIN_REFRESH_TOKEN to src/local.settings.json.example and docs/configuration.md
• Add LinkedInTokenRefreshSchedule App Setting documentation
• Update docs/analysis-linkedin-token-auto-refresh.md: mark Open Questions as resolved, document decisions taken (Key Vault shared, client_secret in KV, cron 0 0 9 1 * *)

See #56 for the full reasoning behind the deferral of the refresh token chain.