Update postgres Docker tag to v18

[renovate]

This PR contains the following updates:

Package	Update	Change
postgres	major	16-alpine → 18-alpine

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[github-actions]

Sonar insights (advisory, no merge-block)

Snapshot of bernstein on the configured Sonar instance:

Metric	Value
Coverage	13.5
Code smells	153
Bugs	11
Vulnerabilities	2
Security hotspots	91

Run bernstein doctor sonar locally for the full surface.

This comment is a soft signal. The Sonar scan runs on push to main; the PR check itself never fails on smells.

[github-actions]

Review-bot acknowledgement summary

• Must-address findings: 0 (0 acknowledged, 0 open)
• Informational findings: 0

All must-address findings are resolved or acknowledged.

[github-actions]

bernstein doctor observe for PR #1903 (renovate/postgres-18.x): ok=1, warn=1, fail=0, error=0, skipped=2

sonar -- WARN (project bernstein)

metric	value	delta	threshold	status
coverage_pct	13.5%	new	80.0%	fail
code_smells	153	new	50	warn
bugs	11	new	0	fail
vulnerabilities	2	new	0	warn
security_hotspots	91	new	0	fail

code-scanning -- OK (18 open alert(s))

metric	value	delta	threshold	status
open_alerts	18	new	0	fail
critical_alerts	0	new	0	ok
high_alerts	0	new	0	ok
medium_alerts	0	new	-	ok
low_alerts	0	new	-	ok

Skipped backends (credentials not configured)

• glitchtip: BERNSTEIN_GLITCHTIP_TOKEN not set
• dt: DTRACK_URL/TOKEN/PROJECT not set

See docs/observability/unified-doctor.md for backend setup notes.