feat: extract .env validators to common.sh and add validate_env()

Move 10 validation functions from configure_env.sh to lib/common.sh as
shared utilities. Define variable-to-validator mapping using parallel
arrays (bash 3.2 compatible). validate_env() checks all ~50 .env
variables against their expected format and reports all failures at once.

Wired into preflight.sh (Check 6b) and bitwarden_to_env.sh (post-restore).
configure_env.sh now sources validators from common.sh instead of
defining its own copies.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -65,7 +65,7 @@ Each phase has three scripts: the main script, a `_post_check.sh` that independe
 gitea-migration/
 ├── .env.example              # Configuration template (copy to .env)
 ├── runners.conf.example      # Runner definitions template
-├── lib/common.sh             # Shared functions (535 lines)
+├── lib/common.sh             # Shared functions + .env validators
 ├── setup/
 │   ├── configure_env.sh      # Interactive .env wizard (~50 prompts)
 │   ├── macbook.sh            # Local prerequisites (brew packages)
@@ -87,7 +87,7 @@ gitea-migration/
 ├── backup/
 │   ├── backup_primary.sh     # Gitea dump, SCP to Fedora
 │   └── restore_to_primary.sh # Restore dump to Unraid
-├── preflight.sh              # 24 pre-flight validation checks
+├── preflight.sh              # 25 pre-flight validation checks
 ├── run_all.sh                # Full pipeline orchestration
 ├── teardown_all.sh           # Reverse teardown (9 to 1)
 ├── manage_runner.sh          # Dynamic runner add/remove/list