refactor: split repo.rs God file into sidecar + per-theme test modules

[StudentWeis]

Current State

src/repository/repo.rs is currently a 1,694-line file. Roughly 460 lines are production logic; the remaining ~1,300 lines are #[cfg(test)] test cases embedded in the same file. The file simultaneously handles repository construction, schema migration, save/get/delete/clear, pin logic, dedup, and rich-text/image sidecar file management.

Target State

• Tests are split by theme into src/repository/tests/{save,pin,dedup,display}_tests.rs, mounted via #[cfg(test)] mod tests; in src/repository/mod.rs.
• Filesystem sidecar helpers (remove_image_files, remove_record_sidecars, remove_superseded_rich_text_files, schema-migration / clear directory cleanup) live in a new src/repository/sidecar.rs as free functions.
• repo.rs is responsible only for DB encode/decode and delegates filesystem side-effects to sidecar.

Rationale

• A single test edit currently recompiles the entire 1.7k-line file → slow incremental builds.
• Adding a new ContentType requires reading the whole file to feel safe — high cognitive overhead.
• Mixing DB logic with filesystem side-effects makes both harder to reason about and test in isolation.

This is item §1.1 ("src/repository/repo.rs — God File ⚠️ High Priority") in doc/CODE_REVIEW.md.

Acceptance Criteria

• repo.rs production code stays ≤ ~500 lines and contains no embedded test module.
• src/repository/sidecar.rs exists and owns all sidecar filesystem helpers as free functions.
• Tests live under src/repository/tests/{save,pin,dedup,display}_tests.rs.
• scripts/precheck.sh passes (fmt / check / clippy -D warnings / test / machete / i18n / icons / themes).
• No behavior change — all existing tests still pass without modification of their assertions.

Scope

repository