refactor(store): single account name on users; rename profile/laps tables

users.display_name becomes users.name and is now the only human-facing
name -- profiles.name is dropped (it duplicated the account name; the
Profile page's Name field now edits users.name through PUT /api/profile,
which carries Name alongside the profiles columns). The profile table
becomes profiles and laps becomes activity_laps, homogeneous with
activity_samples. Onboarding pre-fills the display name from the OIDC
claim's name.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/store/schema.sql

@@ -8,26 +8,26 @@
 -- here rather than appending an ALTER TABLE migration.
 
 -- One geniusrun account per OIDC subject. Every other table below is scoped
--- to a user_id, directly (profile/workout_kinds/activities/sync_state/
--- sync_runs) or transitively through a JOIN to the owning row (laps/
+-- to a user_id, directly (profiles/workout_kinds/activities/sync_state/
+-- sync_runs) or transitively through a JOIN to the owning row (activity_laps/
 -- activity_samples/kind_assignments/workout_type_paces, which have no
 -- user_id column of their own since they're never queried except through a
--- specific activity or workout kind).
+-- specific activity or workout kind). name is the account's single
+-- human-facing name: set at onboarding, editable from the Profile page.
 CREATE TABLE users (
-    id           INTEGER PRIMARY KEY AUTOINCREMENT,
-    oidc_sub     TEXT NOT NULL UNIQUE,
-    display_name TEXT NOT NULL,
-    created_at   TEXT NOT NULL DEFAULT (datetime('now'))
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    oidc_sub   TEXT NOT NULL UNIQUE,
+    name       TEXT NOT NULL,
+    created_at TEXT NOT NULL DEFAULT (datetime('now'))
 );
 
 -- One row per user: Garmin credentials plus every tunable analysis-engine
 -- parameter (HR zones, phase-detection minutes, pace-artifact filtering,
 -- chart colors). store.ProvisionUser creates this (and everything else
 -- below) in one transaction when a new account signs up.
-CREATE TABLE profile (
+CREATE TABLE profiles (
     id                  INTEGER PRIMARY KEY AUTOINCREMENT,
     user_id             INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
-    name                TEXT NOT NULL DEFAULT 'Default',
     garmin_email        TEXT NOT NULL DEFAULT '',
     garmin_password     TEXT NOT NULL DEFAULT '',
     -- Set once, the first time this user successfully authenticates with
@@ -168,7 +168,7 @@ CREATE INDEX idx_activities_start_time ON activities(start_time_utc);
 -- One row per lap/split (from get_activity_splits), plus HR drift/recovery
 -- derived from activity_samples. No user_id column -- always accessed
 -- through a specific owning activity.
-CREATE TABLE laps (
+CREATE TABLE activity_laps (
     id                      INTEGER PRIMARY KEY AUTOINCREMENT,
     activity_id             INTEGER NOT NULL REFERENCES activities(id) ON DELETE CASCADE,
     lap_index               INTEGER NOT NULL,