feat(db): add Flyway migrations — schema V1 and demo seed V2

V1 creates users, projects, time_entries with CHECK constraints and
three indexes (entry_date, project_id, user+date composite).
V2 seeds 3 users (Alice/Bob/Clara) and 3 projects, one with a
project-level hourly rate override (Acme Relaunch).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V1__init.sql

@@ -0,0 +1,26 @@
+CREATE TABLE users (
+    id                        BIGSERIAL    PRIMARY KEY,
+    name                      TEXT         NOT NULL,
+    default_hourly_rate_cents INTEGER      NOT NULL CHECK (default_hourly_rate_cents >= 0)
+);
+
+CREATE TABLE projects (
+    id                   BIGSERIAL    PRIMARY KEY,
+    name                 TEXT         NOT NULL,
+    time_budget_minutes  INTEGER      NOT NULL CHECK (time_budget_minutes > 0),
+    cost_budget_cents    INTEGER      NOT NULL CHECK (cost_budget_cents >= 0),
+    -- NULL means: fall back to the logging user's default_hourly_rate_cents
+    hourly_rate_cents    INTEGER      NULL     CHECK (hourly_rate_cents IS NULL OR hourly_rate_cents >= 0)
+);
+
+CREATE TABLE time_entries (
+    id               BIGSERIAL    PRIMARY KEY,
+    user_id          BIGINT       NOT NULL REFERENCES users(id),
+    project_id       BIGINT       NOT NULL REFERENCES projects(id),
+    entry_date       DATE         NOT NULL,
+    duration_minutes INTEGER      NOT NULL CHECK (duration_minutes > 0 AND duration_minutes <= 1440)
+);
+
+CREATE INDEX idx_time_entries_entry_date  ON time_entries (entry_date);
+CREATE INDEX idx_time_entries_project_id  ON time_entries (project_id);
+CREATE INDEX idx_time_entries_user_date   ON time_entries (user_id, entry_date);

backend/src/main/resources/db/migration/V2__seed_demo_data.sql

@@ -0,0 +1,10 @@
+INSERT INTO users (name, default_hourly_rate_cents) VALUES
+    ('Alice Müller',  9500),  -- 95.00 EUR/h default rate
+    ('Bob Schmidt',   8000),  -- 80.00 EUR/h default rate
+    ('Clara Weber',  11000);  -- 110.00 EUR/h default rate
+
+-- Project A: uses per-project rate override (120 EUR/h)
+INSERT INTO projects (name, time_budget_minutes, cost_budget_cents, hourly_rate_cents) VALUES
+    ('Acme Relaunch',      12000,  150000000, 12000),  -- 200h budget, 1500 EUR budget, 120 EUR/h
+    ('Internal Tools',      6000,   40000000,  NULL),  -- 100h budget, 400 EUR budget, user rate
+    ('Support & Ops',       3000,   24000000,  NULL);  -- 50h budget, 240 EUR budget, user rate