fix(dev): mount build.rs into cryptify-fileshare so PG_CORE_VERSION compiles

[rubenhensen]

Summary

• cryptify/build.rs reads Cargo.lock to extract the pg-core version and exposes it as PG_CORE_VERSION, which src/email.rs:37 consumes via env!() to stamp the X-PostGuard mail header that the Outlook add-in's OnMessageRead filter relies on.
• The cryptify-fileshare dev service only mounted src/ and templates/, so cargo watch -x run saw a package with no build script and failed with environment variable PG_CORE_VERSION not defined at compile time.
• Mount build.rs, Cargo.toml, and Cargo.lock read-only so the build script runs at container startup, and a future pg-core bump in Cargo.lock retriggers the rebuild via cargo:rerun-if-changed=Cargo.lock.

Test plan

• docker compose up -d --build cryptify-fileshare
• docker compose logs -f cryptify-fileshare — confirm cargo builds without the PG_CORE_VERSION error and the service comes up healthy
• Send a test mail via the dev stack and confirm the X-PostGuard header carries the pg-core version from Cargo.lock

[dobby-coder]

Dobby has received the request! Routing to the right specialist now...

[dobby-coder]

Code review

PR is mostly redundant — the build.rs and Cargo.lock mounts already landed on main via #246 (d52ee5b). mergeable=CONFLICTING. Only the Cargo.toml mount is genuinely new. Rebase, drop the duplicates, and update title/body to reflect that only the Cargo.toml sync remains.

Rule compliance

Two rule issues: the PR body over-justifies what reduces to a one-line change post-rebase (no-justification-paragraphs-for-simple-changes), and the conflicting state needs the git diff origin/main --stat no-op check after resolution (conflict-resolution-check-noop).

[dobby-coder]

[Code review] build.rs (line 21) and Cargo.lock (line 23) mounts duplicate work already on main via #246 (d52ee5b, 2026-06-03, which also bumped cryptify to 63066a1). Only the Cargo.toml mount on line 22 is genuinely new. Rebase onto main and drop these two lines.

[dobby-coder]

[Code review] PR branch's cryptify submodule pointer is 4c30e53, which predates the addition of build.rs upstream (cryptify#170, present from 7816df8). A fresh gh pr checkout 248 && git submodule update --init leaves ./cryptify/build.rs missing, so the bind mount has no source file and Docker will create an empty path or refuse to start — the stated test plan can't be executed against the branch as-shipped. Bump the submodule pointer or rebase onto main (already at 63066a1).

[dobby-coder]

[Rule: no-justification-paragraphs-for-simple-changes] After rebasing onto main (which already has the build.rs and Cargo.lock mounts), this PR collapses to a single added line. The three-bullet rationale in the PR body is more explanation than the change warrants — trim to one or two bullets and let the diff speak.