feat(t-paliad-186): mig 088 — fristenrechner-category trigger

Phase 3 Slice 5 Step F-2. BEFORE INSERT/UPDATE trigger on
paliad.projects rejects any write that binds proceeding_type_id to a
non-fristenrechner-category proceeding_types row. NULL is allowed.

PostgreSQL CHECK constraints can't reference other tables, so this
is the only way to evaluate the (proceeding_types.category =
'fristenrechner') predicate per row without restructuring the
existing FK relationship.

Trigger trades narrower FK + partial-unique-index approach for
keeping the existing schema reference (mig 027) untouched. Slice 9
or later may drop this trigger when the litigation category is
fully retired.

Error message is bilingual (German + English) so the Go handler can
either surface it verbatim OR — preferably — intercept the typed
service error first and emit a clean i18n string. mig 088 is
defence-in-depth; the Go service-layer validation is the primary
path.

Idempotent: CREATE OR REPLACE FUNCTION + DROP TRIGGER IF EXISTS
before CREATE TRIGGER.

internal/db/migrations/088_project_proceeding_type_check.down.sql

@@ -0,0 +1,5 @@
+-- t-paliad-186 down — reverses 088_project_proceeding_type_check.up.sql.
+
+DROP TRIGGER IF EXISTS projects_proceeding_type_category_check
+    ON paliad.projects;
+DROP FUNCTION IF EXISTS paliad.projects_proceeding_type_category_check();

internal/db/migrations/088_project_proceeding_type_check.up.sql

@@ -0,0 +1,90 @@
+-- t-paliad-186 / Fristen Phase 3 Slice 5 Step F-2 — enforce
+-- "fristenrechner-category only" on paliad.projects.proceeding_type_id
+-- via a BEFORE INSERT/UPDATE trigger. PostgreSQL CHECK constraints
+-- can't reference other tables, so a trigger is the only way to
+-- evaluate the (proceeding_types.category = 'fristenrechner')
+-- predicate per row.
+--
+-- Why trigger over deferrable-FK-to-partial-index: a partial unique
+-- index on proceeding_types where category='fristenrechner' would
+-- let us reference it from a separate FK column, but the existing
+-- FK on projects.proceeding_type_id → proceeding_types.id is
+-- broad-category. Replacing it with a narrower FK would invalidate
+-- the existing schema reference in mig 027. A trigger keeps the FK
+-- in place and just adds the category predicate on top.
+--
+-- Behaviour:
+--   - INSERT/UPDATE with proceeding_type_id IS NULL: pass (NULL is allowed).
+--   - INSERT/UPDATE with proceeding_type_id pointing at a
+--     fristenrechner-category row: pass.
+--   - INSERT/UPDATE with proceeding_type_id pointing at any other
+--     category: RAISE EXCEPTION with a German + English message so the
+--     handler / frontend can surface a friendly error.
+--   - INSERT/UPDATE with proceeding_type_id pointing at a missing row:
+--     the existing FK on the column rejects it before this trigger
+--     even fires; nothing to do here.
+--
+-- Removed when the litigation category is fully retired (Slice 9 or
+-- later). Until then this is the runtime guard for any writer that
+-- bypasses the Go service-layer validation.
+--
+-- Idempotent: re-applying the migration drops + recreates the trigger.
+
+CREATE OR REPLACE FUNCTION paliad.projects_proceeding_type_category_check()
+RETURNS trigger
+LANGUAGE plpgsql
+AS $$
+DECLARE
+    v_category text;
+BEGIN
+    IF NEW.proceeding_type_id IS NULL THEN
+        RETURN NEW;
+    END IF;
+
+    SELECT category INTO v_category
+      FROM paliad.proceeding_types
+     WHERE id = NEW.proceeding_type_id;
+
+    -- The FK on the column guarantees v_category is non-NULL when the
+    -- id resolves — but defensive against a future FK relax-and-replace.
+    IF v_category IS NULL THEN
+        RAISE EXCEPTION
+            'paliad.projects.proceeding_type_id = % does not resolve to a '
+            'proceeding_types row — FK constraint should have caught this.',
+            NEW.proceeding_type_id;
+    END IF;
+
+    IF v_category <> 'fristenrechner' THEN
+        RAISE EXCEPTION
+            'paliad.projects.proceeding_type_id must reference a '
+            'fristenrechner-category proceeding_types row (got category=''%''). '
+            'Verfahrenstyp muss ein Fristenrechner-Typ sein (Kategorie=''%''). '
+            'Slice 5 (Phase 3 soft-merge per design §3.F) retires the '
+            '''litigation'' category for project-binding; pick a UPC_*, '
+            'DE_*, EPA_*, DPMA_* or EP_GRANT code instead.',
+            v_category, v_category;
+    END IF;
+
+    RETURN NEW;
+END;
+$$;
+
+COMMENT ON FUNCTION paliad.projects_proceeding_type_category_check() IS
+    'BEFORE INSERT/UPDATE trigger function enforcing the Phase 3 Slice 5 '
+    'invariant: paliad.projects.proceeding_type_id may only reference '
+    'fristenrechner-category proceeding_types rows. NULL is allowed.';
+
+DROP TRIGGER IF EXISTS projects_proceeding_type_category_check
+    ON paliad.projects;
+
+CREATE TRIGGER projects_proceeding_type_category_check
+    BEFORE INSERT OR UPDATE OF proceeding_type_id ON paliad.projects
+    FOR EACH ROW
+    EXECUTE FUNCTION paliad.projects_proceeding_type_category_check();
+
+COMMENT ON TRIGGER projects_proceeding_type_category_check ON paliad.projects IS
+    'Phase 3 Slice 5 (t-paliad-186) runtime guard for the projects '
+    'soft-merge — rejects any INSERT/UPDATE that would bind a project '
+    'to a non-fristenrechner-category proceeding_type. The Go service '
+    'layer also enforces this with a typed error; this trigger is the '
+    'defence-in-depth backstop.';