fix(postgresql): validate PITR retention TTL

[weicao]

Summary

• require a configured PITR retention TTL to contain only decimal digits
• reject malformed TTL before interval and expiry arithmetic
• leave repository objects, backup metadata, and the purge checkpoint untouched on invalid configuration

TDD evidence

• RED test commit: e74e77a4e - 59 examples, 1 failure with 5 failed assertions
• GREEN fix commit: 500684b7a - focused 59 examples, 0 failures
• PostgreSQL ShellSpec with Bash 5: 272 examples, 0 failures
• ShellCheck severity error: clean
• Bash 5 syntax: clean
• git diff --check: clean
• helm lint addons/postgresql: 1 chart, 0 failures
• Helm render: 41 documents, 1,012,445 bytes

Behavior

Previously DP_TTL_SECONDS=not-a-number was interpreted as shell arithmetic variables, yielding the current epoch as the expiration cutoff. The script then deleted WALs, published metadata, advanced the checkpoint, and returned success. Invalid TTL now fails before any repository access.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (a758cda) to head (500684b).

Files with missing lines	Patch %	Lines
...ons/postgresql/scripts-ut-spec/pitr_backup_spec.sh	0.00%	11 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##           easton/pg-pitr-empty-purge-checkpoint   #3390   +/-   ##
=====================================================================
  Coverage                                   0.00%   0.00%           
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  Files                                        195     195           
  Lines                                      28256   28267   +11     
=====================================================================
- Misses                                     28256   28267   +11     

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