Sources: per-source paywall override (3-state) — fix domain-rule mis-flags

The Articles inspector revealed paywall is domain-coarse: nytimes.com is flagged,
so NY Times Learning's free Word-of-the-Day inherits 🔒 — and that flag isn't
cosmetic, it deprioritizes the content in feed sort + lead selection. Add a
per-source override so admins can correct it after inspecting.

- sources.paywall_override: NULL (domain rule) | 'free' | 'paywalled'.
- paywall.py: keep low-level is_paywalled(url) (domain); add is_paywalled_for_source
  (url, override) for the EFFECTIVE decision — never patched the domain helper
  globally (per Codex), so "domain says X" stays distinguishable from "overridden".
- Threaded everywhere ranking/UI touches paywall, via src.paywall_override on the
  shared _ARTICLE_COLUMNS + the source-aware helper: feed sort, /api/since, replace,
  lead selection, Article badge, brief composition (briefs.py), digest, source_health
  (table 🔒), the Articles inspector, and the review/attention check — so ranking and
  UI always agree.
- Endpoint POST /api/admin/sources/{id}/paywall {override}; admin UI: a select in the
  inspector header (Use domain rule / Treat as free / Treat as paywalled) + the basis
  ("ON (domain)" / "OFF (override)"), optimistic so the panel stays open.

Test: domain rule → paywalled in table+inspector+feed badge; 'free' → off in all
three; validation 422 + 404. 242 pytest + 11 vitest.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

goodnews/db.py

@@ -392,6 +392,7 @@ def _migrate(conn: sqlite3.Connection) -> None:
         "consecutive_failures": "INTEGER NOT NULL DEFAULT 0",
         "review_flag": "INTEGER NOT NULL DEFAULT 0",
         "review_reason": "TEXT",
+        "paywall_override": "TEXT",  # NULL = use domain rule · 'free' · 'paywalled'
     }
     for column, decl in health_columns.items():
         if column not in source_cols: