Revise ColdFront architecture diagram in README

[zaidshabbir25]

Updated the architecture diagram for ColdFront to clarify the hot and cold tier structure and their components.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Repository: pgEdge/coldfront/.coderabbit.yaml
• Review profile: CHILL
• Plan: Advanced
• Run ID: e1c7106f-b6cc-4dbb-b040-555500c1d1d8

📥 Commits

Reviewing files that changed from the base of the PR and between ea4cc92 and a28c70a.

📒 Files selected for processing (2)

• README.md
• docs/index.md

🚧 Files skipped from review as they are similar to previous changes (2)

• docs/index.md
• README.md

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📝 Walkthrough

Walkthrough

The README and documentation index diagrams show application access through one events relation, PostgreSQL hot and cold storage paths, and the Go archiver moving partitions from hot to cold on a cron schedule.

Changes

Storage-tier architecture diagrams

Layer / File(s)	Summary
Document the storage-tier architecture README.md, docs/index.md	Both diagrams show the application using events, PostgreSQL 16/17/18 routing writes by tier, and separate hot and cold storage components. They identify pg_duckdb, Lakekeeper, object storage, and the scheduled Go archiver flow.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to a28c7

This PR only updates the architecture diagrams in the README and documentation index. It has no runtime impact and is safe to merge.

Architecture Summary

Architecture risk: 🔵 Low · up to ea4cc

The change affects 2 systems.

Changed systems: docs, README.md

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — docs (service) was modified; 1 changed file maps to changed impact.
• observed — README.md (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in README.md: Replaces the compact tier diagram with a labeled flow from application statements on events through PostgreSQL to hot and cold tiers. The new diagram identifies _events as native range partitions, pg_duckdb as in-process DuckDB for Iceberg reads and writes, Lakekeeper as the REST catalog, and the object store as holding Parquet data and Iceberg metadata. It depicts the Go archiver moving data from hot storage toward the object store and adds a cron-job annotation; the old diagram described it as a Go cron process moving partitions past the hot window.
• observed — Modified behavior in docs/index.md: The diagram now separates the hot and cold paths beneath PostgreSQL: _events stores recent rows in native range partitions, while pg_duckdb handles Iceberg reads and writes through Lakekeeper and object storage. It labels the extension as routing writes by tier and depicts the Go archiver moving partitions from hot to cold on a cron schedule. This replaces the prior compact diagram, which showed the archiver and cron together and represented the tier paths with a simpler layout.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: revising the ColdFront architecture diagram in the README and related documentation.
Description check	✅ Passed	The description accurately states that the architecture diagram was updated to clarify the hot and cold tier structure and its components.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[vyruss]

index.md should be updated - shared sections with README must be identical

[susan-pgedge]

If the diagram looks correct, I'll add the update into index.md...