feat(store): persist summary actions (watch/skip/save) — Stage-0 metric

The summary_actions table records the maintainer's act on a summary, the
column that makes the Stage-0 headline test ("acts on >=1 summary") queryable
(ui-spec.md §5, ADR-011). This is the gate lanes B/C build on.

- Migration 002: summary_actions (id, user_id, video_id TEXT, action, acted_at)
  with a CHECK on action IN ('watched','skipped','saved') and a UNIQUE
  (user_id, video_id, action). Per-user isolation: every row carries user_id.
- New actions.go: SetAction (idempotent, atomic watched<->skipped mutual
  exclusion in one tx; saved independent), ClearAction, ActionsFor for the list
  view, plus Go-side action validation.
- reads.go: additive SummaryRow.Actions, populated by composing ActionsFor
  (Go-side, not a SQL join — summaries.video_id is UUID, actions.video_id TEXT).
- embedded-postgres tests: set/clear, mutual exclusion, saved coexistence,
  idempotency, invalid rejection, user scoping, read-view surfacing.

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

internal/adapters/store/migrations/002_summary_actions.down.sql

@@ -0,0 +1 @@
+DROP TABLE summary_actions;

internal/adapters/store/migrations/002_summary_actions.up.sql

@@ -0,0 +1,19 @@
+-- summary_actions records the maintainer's act on a summary (watch/skip/save) —
+-- the column that makes the Stage-0 headline metric ("acts on >=1 summary")
+-- queryable (ui-spec.md §5, ADR-011). Per-user isolation: every row carries
+-- user_id (dormant authz at Stage 0, single user). video_id is TEXT and not
+-- FK-constrained, mirroring summaries' standalone (user_id, video_id) key.
+
+CREATE TABLE summary_actions (
+    id        UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id   UUID NOT NULL,
+    video_id  TEXT NOT NULL,
+    action    TEXT NOT NULL,
+    acted_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    CONSTRAINT summary_actions_action_check
+        CHECK (action IN ('watched', 'skipped', 'saved')),
+    CONSTRAINT summary_actions_user_video_action_unique
+        UNIQUE (user_id, video_id, action)
+);
+
+CREATE INDEX idx_summary_actions_user_video ON summary_actions(user_id, video_id);