Shared-curriculum sub-modules: umbrella

Design doc: _docs/planning/shared-curriculum-submodules-design.md (committed 7a9816e5) — read it in full before implementing any phase; this issue is a summary and phase tracker, not the source of truth.

Motivation

LLM Zoomcamp's real content already has modules that informally split into parts (module 1: "Part 1: RAG" / "Part 2: Agents" in its README prose; module 3 has five parts; module 4 has two) — but there is currently zero machine-readable representation of this anywhere in the pipeline. module.yaml has a flat units: list, and the website builds a flat SharedModule -> SharedLesson graph with no second level.

Verified in the real content: the current single homework for module 1 spans both parts (Q1-5 are Part 1, Q6 is Part 2) — so the model must support a module keeping ONE homework across its parts, not force a split. Separately, Part 2 already advertises a standalone workshop entry point, so a future cohort anchoring a homework to just one part is realistic, not hypothetical. Both shapes (homework on the whole module, homework on one part) are first-class requirements, not one common case and one rare edge case.

Design summary

• SharedSubModule — a new, flat (one level only, no self-referential tree) model, required FK to SharedModule. Rejected a self-referential parent FK on SharedModule itself: every existing SharedModule query (module page, family syllabus count, sitemap, inventory, the two-segment router) would need a parent__isnull=True guard it doesn't have today, and SharedModule's own constraints (slug/position uniqueness, the cohort-identifier collision check) are root-shaped.
• SharedLesson.sub_module — new nullable FK, validated to belong to the lesson's module. A module with no sub-modules is simply all-None — zero change for the common case.
• Homework anchoring — CohortSharedModule (the per-cohort homework placement) gains a nullable sub_module FK: a homework anchors to either the whole module OR one specific sub-module, per cohort, never both, never neither. Because homework is cohort-owned and the shared module graph isn't, two different cohorts can anchor differently against the same shared content (2026 keeps one module-level homework; a future 2027 could split it) with no change to the shared rows.
• Three DB constraints ship in the first phase (not deferred): one module-level homework per module per cohort, one homework per sub-module per cohort, and each homework anchored at most once — see the design doc section 4 for the exact conditional-unique SQL shape (a plain 3-column unique constraint does NOT work here; Postgres treats NULLs as distinct).
• Course-repository side: module.yaml gains an optional sub_modules: list (nested, exclusive with units:); cohort.yaml homework bindings gain an optional sub_module: <slug>. No schema version bump — additive schema-2 extension, safe because both the parser and the zoomcamp-ops checker already reject unknown keys (that strictness IS the rollout safety net). Folders stay flat; no lesson/image/code file moves — the grouping lives only in module.yaml, matching how authors already number parts in ranges (module 4's 06→11 gap). Full field-by-field YAML shape and checker rule list in the design doc's "Course-repository representation" section.
• llm-zoomcamp's real 2026 content needs zero changes to ship this — all seven of its homework bindings stay module-level anchors. Splitting sub_modules: into module.yaml for modules 1/3/4 is Phase D, separate from and not required by Phases A-C.

Phases (in dependency order)

Open questions (design doc has full detail; flagging here for visibility)

1. Should a module ever be allowed both a module-level AND a sub-module-level homework anchor in the same cohort? Schema permits it (each has a well-defined rendering); if it should be forbidden, that's an opt-in zoomcamp-ops checker rule (mixed_homework_anchors), not a DB constraint. Not enabled by default.
2. Phase D timing — author real sub_modules: content for llm-zoomcamp as soon as B/C ship, or leave the capability built but unused for now?
3. Sub-module pages: this design gives parts headings + fragment anchors on the existing module page, not their own routes/URLs. Confirm that's sufficient (a standalone "Part 2: Agents" workshop page could be added later without touching content, since the sub-module slug is already a stable identity).
4. Sub-module slug/title convention: proposed ordinal-free slugs (rag, agents) with README-matching titles ("Part 1: RAG") — confirm, or prefer ordinal-free titles with the website deriving "Part N" from list position.

Each phase will be filed as its own groomed sub-issue per _docs/PROCESS.md, dependent on the previous phase.

Read _docs/planning/shared-curriculum-submodules-design.md at commit 7a9816e5 in full; this umbrella issue is only a phase tracker. Start with the independently groomed Phase A–D sub-issues in dependency order, and treat completion as shipping each phase with its listed models, validators, rendering, repository, and checker work while resolving the open design questions.