feat: add pg_cron and pg_partman, update all extensions and README

[pyramation]

Summary

Adds pg_cron (v1.6.7) and pg_partman (v5.4.3) to the PostgreSQL Docker image, and brings everything up to date.

New extensions

• pg_cron v1.6.7 — job scheduler for periodic tasks
• pg_partman v5.4.3 — partition management
• Both added to shared_preload_libraries (including pg_partman_bgw for automatic partition maintenance)

Version updates

• pg_textsearch bumped from 0.6.1 → 1.2.0

README improvements

• Added version column to extensions table
• Fixed image name to match CI workflow (ghcr.io/constructive-io/docker/postgres-plus)
• Updated pg_textsearch link to GitHub repo
• Updated "Building manually" section with correct image name

Review & Testing Checklist for Human

• Verify Docker image builds on both amd64 and arm64 (CI does this)
• Run make test to confirm all 5 extensions load
• Spot-check pg_textsearch 1.2.0 works with existing BM25 indexes (major version bump from 0.6.1)

Notes

• pgvector (0.8.2) and PostGIS (3.6.2) were already at latest — no changes needed
• pg_textsearch 1.2.0 uses the same build steps (make / make install) as 0.6.1

Link to Devin session: https://app.devin.ai/sessions/42031dddb1aa40a4b747119fd97cc02a
Requested by: @pyramation

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