Finish exercises refactor: rename, restructure, and fix all examples

[mre]

Completes the in-progress reshuffle of the exercise track to the 19-exercise progression documented in the README (00–18).

Renames / new files

• Renumber and rename exercises so file names match the README progression
  (e.g. 01_integer_handling → 02_numbers_and_math, 11_password_validator →
  12_password_validator, 12_question_mark_operator → 13_question_mark_operator,
  15_rust_fundamentals_quiz → 16_quiz, etc.).
• New foundation exercises: 01_basic_types, 03_strings_and_text,
  04_collections_basics, 08_ownership_and_borrowing, 10_enums_and_matching.

Make every example compile cleanly

• 00_hello_rust: add #[cfg(test)] + use super::*; fix four lines accidentally
  written as ///! (outer doc comment) instead of //! (inner module docs).
• 01–04 (foundation): add missing #[cfg(test)] on mod tests and gate bonus
  modules with #[cfg(all(test, feature = "bonus"))] so they participate in
  the test build instead of being silently skipped.
• 05_tuples_and_structs: mark bonus functions pub and reach them from
  bonus_tests via super::super::bonus so the bonus build compiles.
• 06_option_and_some_none: add explicit lifetime to find_user_by_id.
• 08_ownership_and_borrowing: add explicit lifetime to find_longest_word.
• 13_question_mark_operator: stop comparing Result<_, io::Error> /
  Result<_, Box<dyn Error>> with assert_eq!; unwrap() and compare values.
• 15_modules: rewrite so the modules expose a real pub API and the wrapper
  functions are todo!() placeholders the student fills in.

Cargo / server cleanup

• Cargo.toml: fix stale [package.metadata.exercises.12_question_mark_operator]
  key (now 13_question_mark_operator).
• src/bin/server.rs: silence two dead_code warnings on DbParticipant /
  DbSubmission with #[allow(dead_code)] (sqlx::FromRow needs the fields).

Repo hygiene

• .gitignore: ignore local TODO/ scratch directory so planning notes stay
  out of the tree.

Verification

• cargo test --example XX --no-run is green for all 19 exercises.
• cargo test --example XX --features bonus --no-run is green for all 19.
• cargo build --bin server is warning-free.