Add self-service password change, forgot-password flow, and fix admin UI bugs

Users can change their own password from the profile modal, and a
"forgot password" link sends a 15-minute expiring reset link (same
hashed-token pattern as site invites). The forgot-password response is
always generic so it never reveals which emails are registered.

Also fixes two admin-page display bugs found while testing: table row
divider lines that didn't line up across a row (the actions column had
`display: flex` on the <td> itself, breaking it out of normal table-cell
layout -- moved to a child <div>), and the pending-invites list floating
with no visual grouping (now boxed with a label and per-status badges).

backend/app/services/password_service.py

@@ -0,0 +1,78 @@
+import secrets
+from datetime import datetime, timezone
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models import PasswordReset, User
+from app.security import hash_password, hash_token, verify_password
+from app.services.email_service import send_email
+
+
+class InvalidCurrentPasswordError(Exception):
+    pass
+
+
+class PasswordResetInvalidError(Exception):
+    pass
+
+
+async def change_password(
+    db: AsyncSession, user: User, current_password: str, new_password: str
+) -> None:
+    if not verify_password(current_password, user.password_hash):
+        raise InvalidCurrentPasswordError()
+    user.password_hash = hash_password(new_password)
+    await db.commit()
+
+
+async def request_password_reset(db: AsyncSession, email: str, base_url: str) -> None:
+    # Always returns normally, whether or not the email matched an account --
+    # the router never reveals which, to avoid leaking registered emails.
+    result = await db.execute(
+        select(User).where(User.email == email, User.is_active.is_(True))
+    )
+    user = result.scalar_one_or_none()
+    if user is None:
+        return
+
+    raw_token = secrets.token_urlsafe(32)
+    db.add(PasswordReset(user_id=user.id, token_hash=hash_token(raw_token)))
+    await db.commit()
+
+    reset_link = f"{base_url.rstrip('/')}/reset-password?token={raw_token}"
+    await send_email(
+        db,
+        email,
+        "Reset your KeepItTalking password",
+        f"Someone requested a password reset for this account.\n\n"
+        f"Reset it here:\n{reset_link}\n\n"
+        f"This link expires in 15 minutes. If you didn't request this, "
+        f"you can ignore this email.",
+    )
+
+
+async def _get_valid_reset(db: AsyncSession, token: str) -> PasswordReset:
+    result = await db.execute(
+        select(PasswordReset).where(PasswordReset.token_hash == hash_token(token))
+    )
+    reset = result.scalar_one_or_none()
+    if reset is None or reset.used:
+        raise PasswordResetInvalidError()
+    if reset.expires_at <= datetime.now(timezone.utc):
+        raise PasswordResetInvalidError()
+    return reset
+
+
+async def validate_reset_token(db: AsyncSession, token: str) -> None:
+    await _get_valid_reset(db, token)
+
+
+async def complete_password_reset(db: AsyncSession, token: str, new_password: str) -> User:
+    reset = await _get_valid_reset(db, token)
+    user = await db.get(User, reset.user_id)
+    user.password_hash = hash_password(new_password)
+    reset.used = True
+    await db.commit()
+    await db.refresh(user)
+    return user