Write spec 03 (Databases) tasks file

[HexSleeves]

Question

Spec 03 has answered questions and a full spec (docs/specs/03-spec-databases/) but no 03-tasks-databases.md. Write the tasks file following the structure of 01-tasks-design-patterns-section.md (task groups, proof artifacts, demoable units).

Parent map: #915

[HexSleeves]

Resolved: added `docs/specs/03-spec-databases/03-tasks-databases.md` following the `01-tasks-design-patterns-section.md` structural template (Relevant Files, task groups with Purpose + Proof Artifact(s) + numbered subtasks), incorporating the answered decisions from `03-questions-1-databases.md`.

8 task groups: SQL fundamentals & schema design, SQLAlchemy ORM, Repository pattern (building on 11.2.2), indexes & N+1 query optimization, NoSQL (Redis + document store), integration exercise, interactive quiz, and sidebar/front-matter integration.

PR: #927

[HexSleeves]

Closing - resolved via #927