#26: optional proposed-use-case field on propose-RFC + propose-PR

Adds an optional "What will you be using this for?" capture as a sibling
to the required justification on both propose surfaces, per roadmap #26.

- propose-RFC modal (ProposeModal): optional textarea below the required
  "Why is this RFC needed?" pitch, labeled "What will you be using this
  RFC for? (optional)".
- propose-PR modal (PRModal): optional textarea below the required
  description, labeled "What will you be using this change for?".
- Backend: ProposeBody / OpenPRBody gain an optional `proposed_use_case`
  (NULL/omitted accepted, no min, 8000-char cap matching the existing
  free-text bound). Persisted to a new canonical side table
  `proposed_use_cases` keyed by PR number, mirrored onto the cache
  columns added by migration 021. Returned on the proposal list/detail,
  RFC detail (by slug), and PR detail endpoints.
- Display: ProposalView, RFCView (main only), and PRView render the
  captured use case with a muted "left blank" treatment when NULL.
- migration 021: nullable `proposed_use_case` on cached_rfcs/cached_prs
  plus the reconcile-proof `proposed_use_cases` truth table.
- New vertical test_proposed_use_case_vertical: persists+returns when
  supplied, accepted as NULL/omitted, for both surfaces.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api.py

@@ -51,6 +51,11 @@ class ProposeBody(BaseModel):
     slug: str = Field(min_length=1, max_length=80)
     pitch: str = Field(min_length=1)
     tags: list[str] = Field(default_factory=list)
+    # Roadmap #26: optional "What will you be using this RFC for?" — the
+    # concrete ground-truth use case, distinct from the `pitch`'s abstract
+    # "why is this needed." Optional (NULL/omitted accepted), no minimum,
+    # generous cap matching the pitch's free-text body bound.
+    proposed_use_case: str | None = Field(default=None, max_length=8000)
 
 
 class DeclineBody(BaseModel):
@@ -557,12 +562,36 @@ def make_router(
         ).fetchone()
         if row is None:
             raise HTTPException(404, "Not found")
-        return _serialize_rfc(row)
+        payload = _serialize_rfc(row)
+        # Roadmap #26: surface the optional propose-time use case on the
+        # RFC view. The idea PR closes on merge, but the canonical row in
+        # `proposed_use_cases` persists; look it up by slug (the latest
+        # 'rfc'-scope row for this slug). NULL == "left blank".
+        uc = db.conn().execute(
+            """
+            SELECT use_case FROM proposed_use_cases
+             WHERE scope = 'rfc' AND rfc_slug = ?
+             ORDER BY id DESC LIMIT 1
+            """,
+            (slug,),
+        ).fetchone()
+        payload["proposed_use_case"] = uc["use_case"] if uc else None
+        return payload
 
     # ---------------------------------------------------------------
     # §7.3 / §9.3: pending ideas
     # ---------------------------------------------------------------
 
+    def _proposal_use_case(pr_number: int) -> str | None:
+        """Roadmap #26: read the optional use case for an idea PR from the
+        canonical side table. Returns None when none was supplied (the
+        "left blank" sentinel the frontend renders tastefully)."""
+        row = db.conn().execute(
+            "SELECT use_case FROM proposed_use_cases WHERE scope = 'rfc' AND pr_number = ?",
+            (pr_number,),
+        ).fetchone()
+        return row["use_case"] if row else None
+
     @router.get("/api/proposals")
     async def list_proposals() -> dict[str, Any]:
         rows = db.conn().execute(
@@ -582,6 +611,7 @@ def make_router(
                     "description": r["description"],
                     "opened_by": r["opened_by"],
                     "opened_at": r["opened_at"],
+                    "proposed_use_case": _proposal_use_case(r["pr_number"]),
                 }
                 for r in rows
             ]
@@ -630,6 +660,7 @@ def make_router(
             "opened_at": row["opened_at"],
             "entry": entry_payload,
             "affordances": affordances,
+            "proposed_use_case": _proposal_use_case(pr_number),
         }
 
     # ---------------------------------------------------------------
@@ -706,6 +737,26 @@ def make_router(
         # cache write is idempotent.)
         await cache.refresh_meta_pulls(config, gitea)
 
+        # Roadmap #26: persist the optional use case to the canonical,
+        # reconcile-proof side table keyed by the idea PR number. NULL/
+        # blank simply writes no row (absence == "left blank"). Done after
+        # the refresh so the cache row exists; the mirror onto cached_prs
+        # keeps the cache column in parity for any read that uses it.
+        use_case = (payload.proposed_use_case or "").strip()
+        if use_case:
+            db.conn().execute(
+                """
+                INSERT INTO proposed_use_cases (scope, rfc_slug, pr_number, use_case)
+                VALUES ('rfc', ?, ?, ?)
+                ON CONFLICT(scope, pr_number) DO UPDATE SET use_case = excluded.use_case
+                """,
+                (slug, pr["number"], use_case),
+            )
+            db.conn().execute(
+                "UPDATE cached_prs SET proposed_use_case = ? WHERE pr_kind = 'idea' AND pr_number = ?",
+                (use_case, pr["number"]),
+            )
+
         return {"pr_number": pr["number"], "slug": slug}
 
     # ---------------------------------------------------------------