fix(postgresql): parse PITR retention TTL as decimal

[weicao]

Summary

• parse the already validated DP_TTL_SECONDS value explicitly as base 10 during PITR expiry cutoff arithmetic
• cover leading-zero decimal TTLs so Bash cannot silently reinterpret them as octal

Candidate

• target branch: easton/pg-pitr-retention-ttl-validation
• exact base: 500684b7a6f07e86b16b5d59b1e4c3c6113dcbc2 (PR fix(postgresql): validate PITR retention TTL #3390)
• exact head: cd5ef60fc
• relevant open child PRs targeting the base: none
• other open weicao PostgreSQL PR targeting main: fix(postgresql): stop pgbouncer after setup failure #3331, excluded because it is the independent PgBouncer setup lane

Contract

• kubeblocks-addon-docs/docs/addon-continuous-backup-pitr-chain-integrity-guide.md, pit 3: retention cleanup must keep the advertised recovery window aligned with repository artifacts
• kubeblocks-addon-docs/docs/addon-api/09b-backup-extensions.md: Continuous/PITR time ranges and repository retention must remain explainable and testable

TDD evidence

RED commit 1d5f0a29e:

• DP_TTL_SECONDS=03600
• expected decimal cutoff: 10000 - 3600 = 6400
• observed before fix: cutoff 8080, because Bash parsed 03600 as octal 1920
• focused result: 1 example, 1 failure

GREEN exact head cd5ef60fc:

• focused Bash 3.2: 60 examples, 0 failures
• full PostgreSQL Bash 5.3: 273 examples, 0 failures
• ShellCheck severity error: clean
• Bash syntax and git diff --check: clean
• Helm lint: 1 chart, 0 failures
• Helm render: 41 documents, 1,012,454 bytes

Runtime boundary

This is source-level arithmetic and ShellSpec validation only. Runtime packaging, environment execution, cleanup, and formal acceptance counts remain Test-owned and are not claimed by this PR.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (500684b) to head (cd5ef60).

Files with missing lines	Patch %	Lines
...ons/postgresql/scripts-ut-spec/pitr_backup_spec.sh	0.00%	7 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                           @@
##           easton/pg-pitr-retention-ttl-validation   #3391   +/-   ##
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  Coverage                                     0.00%   0.00%           
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  Files                                          195     195           
  Lines                                        28267   28274    +7     
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- Misses                                       28267   28274    +7     

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