Add plan dates, postponing, backlog and projects

- Tasks get an optional plan date separate from the deadline. Unfinished
  plans roll over in Today; only missed deadlines count as overdue.
- Postpone sheet on every task (tomorrow, weekend, next week, ...) that
  can move either the plan date or the deadline, with undo.
- Backlog view for undated tasks, with suggestions in Today.
- Projects with deadline, notes, progress, next step and "falling
  behind" warnings, shown in Today and in their own view.
- Times and the calendar now follow the plan date (else the deadline).
- Schema v3 migration (backup taken first); export format v3 includes
  projects and still imports older files.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/db.py

@@ -3,7 +3,7 @@ import sqlite3
 from pathlib import Path
 
 DB_PATH = os.environ.get("DB_PATH", "/data/whattodo.db")
-SCHEMA_VERSION = 2
+SCHEMA_VERSION = 3
 
 DEFAULT_CATEGORIES = [
     ("Work", "#2563eb"),
@@ -23,13 +23,26 @@ CREATE TABLE IF NOT EXISTS categories (
     color    TEXT NOT NULL DEFAULT '#6b7280',
     position INTEGER NOT NULL DEFAULT 0
 );
+CREATE TABLE IF NOT EXISTS projects (
+    id          INTEGER PRIMARY KEY,
+    name        TEXT NOT NULL,
+    notes       TEXT NOT NULL DEFAULT '',
+    category_id INTEGER REFERENCES categories(id) ON DELETE SET NULL,
+    due_date    TEXT,
+    done        INTEGER NOT NULL DEFAULT 0,
+    done_at     TEXT,
+    created_at  TEXT NOT NULL,
+    updated_at  TEXT NOT NULL
+);
 CREATE TABLE IF NOT EXISTS tasks (
     id             INTEGER PRIMARY KEY,
     title          TEXT NOT NULL,
     notes          TEXT NOT NULL DEFAULT '',
     category_id    INTEGER REFERENCES categories(id) ON DELETE SET NULL,
+    project_id     INTEGER REFERENCES projects(id) ON DELETE SET NULL,
     priority       INTEGER NOT NULL DEFAULT 0,
     due_date       TEXT,
+    planned_date   TEXT,
     start_time     TEXT,
     end_time       TEXT,
     tags          TEXT NOT NULL DEFAULT '[]',
@@ -96,6 +109,12 @@ def migrate(conn: sqlite3.Connection, version: int) -> None:
         if version < 2:
             conn.execute("ALTER TABLE tasks ADD COLUMN start_time TEXT")
             conn.execute("ALTER TABLE tasks ADD COLUMN end_time TEXT")
+        if version < 3:
+            # The projects table itself is created by SCHEMA before migrating.
+            conn.execute("ALTER TABLE tasks ADD COLUMN planned_date TEXT")
+            conn.execute(
+                "ALTER TABLE tasks ADD COLUMN project_id INTEGER REFERENCES projects(id) ON DELETE SET NULL"
+            )
         conn.execute(
             "UPDATE meta SET value = ? WHERE key = 'schema_version'", (str(SCHEMA_VERSION),)
         )