Fix PR #221 permission blockers

[SaladDay]

Continues PR #221 from a SaladDay-owned branch.

Changes:

• Harden cc-switch config storage path handling against parent components, user symlink components, symlinked sensitive files, and unsafe permission fixes.
• Reject symlink/non-file/multi-hardlink main SQLite database files before opening or migrating.
• Harden backup DB creation and recursive sensitive backup permission checks.
• Align WebDAV/SQLite compatibility with upstream schema v11, including pricing_model and rollup request/pricing model dimensions.

Verification:

• cargo test with isolated HOME / CC_SWITCH_CONFIG_DIR / app config dirs
• cargo fmt --check
• git diff --check
• cargo clippy
• Fresh-eyes subagent review: NO BLOCKERS for the two prior permission blockers.

[SaladDay]

Closing this handoff PR; continuing directly on PR #221 as requested by the repository owner.